StarPunk/docs/design/v1.1.2/feed-enhancements-spec.md

# Feed Enhancements Specification - v1.1.2

## Overview

This specification defines the feed system enhancements for StarPunk v1.1.2, including content negotiation, caching, statistics tracking, and OPML export capabilities.

## Requirements

### Functional Requirements

1. **Content Negotiation**
   - Parse HTTP Accept headers
   - Score format preferences
   - Select optimal format
   - Handle quality factors (q=)

2. **Feed Caching**
   - LRU cache with TTL
   - Format-specific caching
   - Invalidation on changes
   - Memory-bounded storage

3. **Statistics Dashboard**
   - Track feed requests
   - Monitor cache performance
   - Analyze client usage
   - Display trends

4. **OPML Export**
   - Generate OPML 2.0
   - Include all feed formats
   - Add feed metadata
   - Validate output

### Non-Functional Requirements

1. **Performance**
   - Cache hit rate >80%
   - Negotiation <1ms
   - Dashboard load <100ms
   - OPML generation <10ms

2. **Scalability**
   - Bounded memory usage
   - Efficient cache eviction
   - Statistical sampling
   - Async processing

## Content Negotiation

### Design

Content negotiation determines the best feed format based on the client's Accept header.

```python
class ContentNegotiator:
    """HTTP content negotiation for feed formats"""

    # MIME type mappings
    MIME_TYPES = {
        'rss': [
            'application/rss+xml',
            'application/xml',
            'text/xml',
            'application/x-rss+xml'
        ],
        'atom': [
            'application/atom+xml',
            'application/x-atom+xml'
        ],
        'json': [
            'application/json',
            'application/feed+json',
            'application/x-json-feed'
        ]
    }

    def negotiate(self, accept_header: str, available_formats: List[str] = None) -> str:
        """Negotiate best format from Accept header

        Args:
            accept_header: HTTP Accept header value
            available_formats: List of enabled formats (default: all)

        Returns:
            Selected format: 'rss', 'atom', or 'json'
        """
        if not available_formats:
            available_formats = ['rss', 'atom', 'json']

        # Parse Accept header
        accept_types = self._parse_accept_header(accept_header)

        # Score each format
        scores = {}
        for format_name in available_formats:
            scores[format_name] = self._score_format(format_name, accept_types)

        # Select highest scoring format
        if scores:
            best_format = max(scores, key=scores.get)
            if scores[best_format] > 0:
                return best_format

        # Default to RSS if no preference
        return 'rss' if 'rss' in available_formats else available_formats[0]

    def _parse_accept_header(self, accept_header: str) -> List[Dict[str, Any]]:
        """Parse Accept header into list of types with quality"""
        if not accept_header:
            return []

        types = []
        for part in accept_header.split(','):
            part = part.strip()
            if not part:
                continue

            # Split type and parameters
            parts = part.split(';')
            mime_type = parts[0].strip()

            # Parse quality factor
            quality = 1.0
            for param in parts[1:]:
                param = param.strip()
                if param.startswith('q='):
                    try:
                        quality = float(param[2:])
                    except ValueError:
                        quality = 1.0

            types.append({
                'type': mime_type,
                'quality': quality
            })

        # Sort by quality descending
        return sorted(types, key=lambda x: x['quality'], reverse=True)

    def _score_format(self, format_name: str, accept_types: List[Dict]) -> float:
        """Score a format against Accept types"""
        mime_types = self.MIME_TYPES.get(format_name, [])
        best_score = 0.0

        for accept in accept_types:
            accept_type = accept['type']
            quality = accept['quality']

            # Check for exact match
            if accept_type in mime_types:
                best_score = max(best_score, quality)

            # Check for wildcard matches
            elif accept_type == '*/*':
                best_score = max(best_score, quality * 0.1)

            elif accept_type == 'application/*':
                if any(m.startswith('application/') for m in mime_types):
                    best_score = max(best_score, quality * 0.5)

            elif accept_type == 'text/*':
                if any(m.startswith('text/') for m in mime_types):
                    best_score = max(best_score, quality * 0.5)

        return best_score
```

### Accept Header Examples

| Accept Header | Selected Format | Reason |
|--------------|-----------------|--------|
| `application/atom+xml` | atom | Exact match |
| `application/json` | json | JSON match |
| `application/rss+xml, application/atom+xml;q=0.9` | rss | Higher quality |
| `text/html, application/*;q=0.9` | rss | Wildcard match, RSS default |
| `*/*` | rss | No preference, use default |
| (empty) | rss | No header, use default |

## Feed Caching

### Cache Design

```python
from collections import OrderedDict
from dataclasses import dataclass
from datetime import datetime, timedelta
from typing import Optional, Any
import hashlib

@dataclass
class CacheEntry:
    """Single cache entry with metadata"""
    key: str
    content: str
    content_type: str
    created_at: datetime
    expires_at: datetime
    hit_count: int = 0
    size_bytes: int = 0

class FeedCache:
    """LRU cache with TTL for feed content"""

    def __init__(self, max_size: int = 100, default_ttl: int = 300):
        """Initialize cache

        Args:
            max_size: Maximum number of entries
            default_ttl: Default TTL in seconds
        """
        self.max_size = max_size
        self.default_ttl = default_ttl
        self.cache = OrderedDict()
        self.stats = {
            'hits': 0,
            'misses': 0,
            'evictions': 0,
            'invalidations': 0
        }

    def get(self, format: str, limit: int, checksum: str) -> Optional[CacheEntry]:
        """Get cached feed if available and not expired"""
        key = self._make_key(format, limit, checksum)

        if key not in self.cache:
            self.stats['misses'] += 1
            return None

        entry = self.cache[key]

        # Check expiration
        if datetime.now() > entry.expires_at:
            del self.cache[key]
            self.stats['misses'] += 1
            return None

        # Move to end (LRU)
        self.cache.move_to_end(key)

        # Update stats
        entry.hit_count += 1
        self.stats['hits'] += 1

        return entry

    def set(self, format: str, limit: int, checksum: str, content: str,
            content_type: str, ttl: Optional[int] = None):
        """Store feed in cache"""
        key = self._make_key(format, limit, checksum)
        ttl = ttl or self.default_ttl

        # Create entry
        entry = CacheEntry(
            key=key,
            content=content,
            content_type=content_type,
            created_at=datetime.now(),
            expires_at=datetime.now() + timedelta(seconds=ttl),
            size_bytes=len(content.encode('utf-8'))
        )

        # Add to cache
        self.cache[key] = entry

        # Enforce size limit
        while len(self.cache) > self.max_size:
            # Remove oldest (first) item
            evicted_key = next(iter(self.cache))
            del self.cache[evicted_key]
            self.stats['evictions'] += 1

    def invalidate(self, pattern: Optional[str] = None):
        """Invalidate cache entries matching pattern"""
        if pattern is None:
            # Clear all
            count = len(self.cache)
            self.cache.clear()
            self.stats['invalidations'] += count
        else:
            # Clear matching keys
            keys_to_remove = [
                key for key in self.cache
                if pattern in key
            ]
            for key in keys_to_remove:
                del self.cache[key]
                self.stats['invalidations'] += 1

    def _make_key(self, format: str, limit: int, checksum: str) -> str:
        """Generate cache key"""
        return f"feed:{format}:{limit}:{checksum}"

    def get_stats(self) -> Dict[str, Any]:
        """Get cache statistics"""
        total_requests = self.stats['hits'] + self.stats['misses']
        hit_rate = (self.stats['hits'] / total_requests * 100) if total_requests > 0 else 0

        # Calculate memory usage
        total_bytes = sum(entry.size_bytes for entry in self.cache.values())

        return {
            'entries': len(self.cache),
            'max_entries': self.max_size,
            'memory_mb': total_bytes / (1024 * 1024),
            'hit_rate': hit_rate,
            'hits': self.stats['hits'],
            'misses': self.stats['misses'],
            'evictions': self.stats['evictions'],
            'invalidations': self.stats['invalidations']
        }

class ContentChecksum:
    """Generate checksums for cache invalidation"""

    @staticmethod
    def calculate(notes: List[Note], config: Dict) -> str:
        """Calculate checksum based on content state"""
        # Use latest note timestamp and count
        if notes:
            latest_timestamp = max(n.updated_at or n.created_at for n in notes)
            checksum_data = f"{latest_timestamp.isoformat()}:{len(notes)}"
        else:
            checksum_data = "empty:0"

        # Include configuration that affects output
        config_data = f"{config.get('site_name')}:{config.get('site_url')}"

        # Generate hash
        combined = f"{checksum_data}:{config_data}"
        return hashlib.md5(combined.encode()).hexdigest()[:8]
```

### Cache Integration

```python
# In feed route handler
@app.route('/feed.<format>')
def serve_feed(format):
    """Serve feed in requested format"""
    # Content negotiation if format not specified
    if format == 'feed':
        negotiator = ContentNegotiator()
        format = negotiator.negotiate(request.headers.get('Accept'))

    # Get notes and calculate checksum
    notes = get_published_notes()
    checksum = ContentChecksum.calculate(notes, app.config)

    # Check cache
    cached = feed_cache.get(format, limit=50, checksum=checksum)
    if cached:
        return Response(
            cached.content,
            mimetype=cached.content_type,
            headers={'X-Cache': 'HIT'}
        )

    # Generate feed
    if format == 'rss':
        content = rss_generator.generate(notes)
        content_type = 'application/rss+xml'
    elif format == 'atom':
        content = atom_generator.generate(notes)
        content_type = 'application/atom+xml'
    elif format == 'json':
        content = json_generator.generate(notes)
        content_type = 'application/feed+json'
    else:
        abort(404)

    # Cache the result
    feed_cache.set(format, 50, checksum, content, content_type)

    return Response(
        content,
        mimetype=content_type,
        headers={'X-Cache': 'MISS'}
    )
```

## Statistics Dashboard

### Dashboard Design

```python
class SyndicationStats:
    """Collect and analyze syndication statistics"""

    def __init__(self):
        self.requests = defaultdict(int)  # By format
        self.user_agents = defaultdict(int)
        self.generation_times = defaultdict(list)
        self.errors = deque(maxlen=100)

    def record_request(self, format: str, user_agent: str, cached: bool,
                      generation_time: Optional[float] = None):
        """Record feed request"""
        self.requests[format] += 1
        self.user_agents[self._normalize_user_agent(user_agent)] += 1

        if generation_time is not None:
            self.generation_times[format].append(generation_time)
            # Keep only last 1000 times
            if len(self.generation_times[format]) > 1000:
                self.generation_times[format] = self.generation_times[format][-1000:]

    def record_error(self, format: str, error: str):
        """Record feed generation error"""
        self.errors.append({
            'timestamp': datetime.now(),
            'format': format,
            'error': error
        })

    def get_summary(self) -> Dict[str, Any]:
        """Get statistics summary"""
        total_requests = sum(self.requests.values())

        # Calculate format distribution
        format_distribution = {
            format: (count / total_requests * 100) if total_requests > 0 else 0
            for format, count in self.requests.items()
        }

        # Top user agents
        top_agents = sorted(
            self.user_agents.items(),
            key=lambda x: x[1],
            reverse=True
        )[:10]

        # Generation time stats
        time_stats = {}
        for format, times in self.generation_times.items():
            if times:
                sorted_times = sorted(times)
                time_stats[format] = {
                    'avg': sum(times) / len(times),
                    'p50': sorted_times[len(times) // 2],
                    'p95': sorted_times[int(len(times) * 0.95)],
                    'p99': sorted_times[int(len(times) * 0.99)]
                }

        return {
            'total_requests': total_requests,
            'format_distribution': format_distribution,
            'top_user_agents': top_agents,
            'generation_times': time_stats,
            'recent_errors': list(self.errors)
        }

    def _normalize_user_agent(self, user_agent: str) -> str:
        """Normalize user agent for grouping"""
        if not user_agent:
            return 'Unknown'

        # Common patterns
        patterns = [
            (r'Feedly', 'Feedly'),
            (r'Inoreader', 'Inoreader'),
            (r'NewsBlur', 'NewsBlur'),
            (r'Tiny Tiny RSS', 'Tiny Tiny RSS'),
            (r'FreshRSS', 'FreshRSS'),
            (r'NetNewsWire', 'NetNewsWire'),
            (r'Feedbin', 'Feedbin'),
            (r'bot|Bot|crawler|Crawler', 'Bot/Crawler'),
            (r'Mozilla.*Firefox', 'Firefox'),
            (r'Mozilla.*Chrome', 'Chrome'),
            (r'Mozilla.*Safari', 'Safari')
        ]

        import re
        for pattern, name in patterns:
            if re.search(pattern, user_agent):
                return name

        return 'Other'
```

### Dashboard Template

```html
<!-- templates/admin/syndication.html -->
{% extends "admin/base.html" %}

{% block title %}Syndication Dashboard{% endblock %}

{% block content %}
<div class="syndication-dashboard">
    <h2>Syndication Statistics</h2>

    <!-- Overview Cards -->
    <div class="stats-grid">
        <div class="stat-card">
            <h3>Total Requests</h3>
            <p class="stat-value">{{ stats.total_requests }}</p>
        </div>
        <div class="stat-card">
            <h3>Cache Hit Rate</h3>
            <p class="stat-value">{{ cache_stats.hit_rate|round(1) }}%</p>
        </div>
        <div class="stat-card">
            <h3>Active Formats</h3>
            <p class="stat-value">{{ stats.format_distribution|length }}</p>
        </div>
        <div class="stat-card">
            <h3>Cache Memory</h3>
            <p class="stat-value">{{ cache_stats.memory_mb|round(2) }}MB</p>
        </div>
    </div>

    <!-- Format Distribution -->
    <div class="chart-container">
        <h3>Format Distribution</h3>
        <canvas id="format-chart"></canvas>
    </div>

    <!-- Top User Agents -->
    <div class="table-container">
        <h3>Top Feed Readers</h3>
        <table>
            <thead>
                <tr>
                    <th>Reader</th>
                    <th>Requests</th>
                    <th>Percentage</th>
                </tr>
            </thead>
            <tbody>
                {% for agent, count in stats.top_user_agents %}
                <tr>
                    <td>{{ agent }}</td>
                    <td>{{ count }}</td>
                    <td>{{ (count / stats.total_requests * 100)|round(1) }}%</td>
                </tr>
                {% endfor %}
            </tbody>
        </table>
    </div>

    <!-- Generation Performance -->
    <div class="table-container">
        <h3>Generation Performance</h3>
        <table>
            <thead>
                <tr>
                    <th>Format</th>
                    <th>Avg (ms)</th>
                    <th>P50 (ms)</th>
                    <th>P95 (ms)</th>
                    <th>P99 (ms)</th>
                </tr>
            </thead>
            <tbody>
                {% for format, times in stats.generation_times.items() %}
                <tr>
                    <td>{{ format|upper }}</td>
                    <td>{{ (times.avg * 1000)|round(1) }}</td>
                    <td>{{ (times.p50 * 1000)|round(1) }}</td>
                    <td>{{ (times.p95 * 1000)|round(1) }}</td>
                    <td>{{ (times.p99 * 1000)|round(1) }}</td>
                </tr>
                {% endfor %}
            </tbody>
        </table>
    </div>

    <!-- Recent Errors -->
    {% if stats.recent_errors %}
    <div class="error-log">
        <h3>Recent Errors</h3>
        <ul>
            {% for error in stats.recent_errors[-10:] %}
            <li>
                <span class="timestamp">{{ error.timestamp|timeago }}</span>
                <span class="format">{{ error.format }}</span>
                <span class="error">{{ error.error }}</span>
            </li>
            {% endfor %}
        </ul>
    </div>
    {% endif %}

    <!-- Feed URLs -->
    <div class="feed-urls">
        <h3>Available Feeds</h3>
        <ul>
            <li>RSS: <code>{{ url_for('serve_feed', format='rss', _external=True) }}</code></li>
            <li>ATOM: <code>{{ url_for('serve_feed', format='atom', _external=True) }}</code></li>
            <li>JSON: <code>{{ url_for('serve_feed', format='json', _external=True) }}</code></li>
            <li>OPML: <code>{{ url_for('export_opml', _external=True) }}</code></li>
        </ul>
    </div>
</div>

<script>
// Format distribution pie chart
const ctx = document.getElementById('format-chart').getContext('2d');
new Chart(ctx, {
    type: 'pie',
    data: {
        labels: {{ stats.format_distribution.keys()|list|tojson }},
        datasets: [{
            data: {{ stats.format_distribution.values()|list|tojson }},
            backgroundColor: ['#FF6384', '#36A2EB', '#FFCE56']
        }]
    }
});
</script>
{% endblock %}
```

## OPML Export

### OPML Generator

```python
from xml.etree.ElementTree import Element, SubElement, tostring
from xml.dom import minidom

class OPMLGenerator:
    """Generate OPML 2.0 feed list"""

    def __init__(self, site_url: str, site_name: str, owner_name: str = None,
                 owner_email: str = None):
        self.site_url = site_url.rstrip('/')
        self.site_name = site_name
        self.owner_name = owner_name
        self.owner_email = owner_email

    def generate(self, include_formats: List[str] = None) -> str:
        """Generate OPML document

        Args:
            include_formats: List of formats to include (default: all enabled)

        Returns:
            OPML 2.0 XML string
        """
        if not include_formats:
            include_formats = ['rss', 'atom', 'json']

        # Create root element
        opml = Element('opml', version='2.0')

        # Add head
        head = SubElement(opml, 'head')
        SubElement(head, 'title').text = f"{self.site_name} Feeds"
        SubElement(head, 'dateCreated').text = datetime.now(timezone.utc).strftime(
            '%a, %d %b %Y %H:%M:%S %z'
        )
        SubElement(head, 'dateModified').text = datetime.now(timezone.utc).strftime(
            '%a, %d %b %Y %H:%M:%S %z'
        )

        if self.owner_name:
            SubElement(head, 'ownerName').text = self.owner_name
        if self.owner_email:
            SubElement(head, 'ownerEmail').text = self.owner_email

        # Add body with outlines
        body = SubElement(opml, 'body')

        # Add feed outlines
        if 'rss' in include_formats:
            SubElement(body, 'outline',
                      type='rss',
                      text=f"{self.site_name} - RSS Feed",
                      title=f"{self.site_name} - RSS Feed",
                      xmlUrl=f"{self.site_url}/feed.xml",
                      htmlUrl=self.site_url)

        if 'atom' in include_formats:
            SubElement(body, 'outline',
                      type='atom',
                      text=f"{self.site_name} - ATOM Feed",
                      title=f"{self.site_name} - ATOM Feed",
                      xmlUrl=f"{self.site_url}/feed.atom",
                      htmlUrl=self.site_url)

        if 'json' in include_formats:
            SubElement(body, 'outline',
                      type='json',
                      text=f"{self.site_name} - JSON Feed",
                      title=f"{self.site_name} - JSON Feed",
                      xmlUrl=f"{self.site_url}/feed.json",
                      htmlUrl=self.site_url)

        # Convert to pretty XML
        rough_string = tostring(opml, encoding='unicode')
        reparsed = minidom.parseString(rough_string)
        return reparsed.toprettyxml(indent='  ', encoding='UTF-8').decode('utf-8')
```

### OPML Example Output

```xml
<?xml version="1.0" encoding="UTF-8"?>
<opml version="2.0">
  <head>
    <title>StarPunk Notes Feeds</title>
    <dateCreated>Mon, 25 Nov 2024 12:00:00 +0000</dateCreated>
    <dateModified>Mon, 25 Nov 2024 12:00:00 +0000</dateModified>
    <ownerName>John Doe</ownerName>
    <ownerEmail>john@example.com</ownerEmail>
  </head>
  <body>
    <outline type="rss"
             text="StarPunk Notes - RSS Feed"
             title="StarPunk Notes - RSS Feed"
             xmlUrl="https://example.com/feed.xml"
             htmlUrl="https://example.com"/>
    <outline type="atom"
             text="StarPunk Notes - ATOM Feed"
             title="StarPunk Notes - ATOM Feed"
             xmlUrl="https://example.com/feed.atom"
             htmlUrl="https://example.com"/>
    <outline type="json"
             text="StarPunk Notes - JSON Feed"
             title="StarPunk Notes - JSON Feed"
             xmlUrl="https://example.com/feed.json"
             htmlUrl="https://example.com"/>
  </body>
</opml>
```

## Testing Strategy

### Content Negotiation Tests

```python
def test_content_negotiation():
    """Test Accept header parsing and format selection"""
    negotiator = ContentNegotiator()

    # Test exact matches
    assert negotiator.negotiate('application/atom+xml') == 'atom'
    assert negotiator.negotiate('application/feed+json') == 'json'
    assert negotiator.negotiate('application/rss+xml') == 'rss'

    # Test quality factors
    assert negotiator.negotiate('application/atom+xml;q=0.8, application/rss+xml') == 'rss'

    # Test wildcards
    assert negotiator.negotiate('*/*') == 'rss'  # Default
    assert negotiator.negotiate('application/*') == 'rss'  # First application type

    # Test no preference
    assert negotiator.negotiate('') == 'rss'
    assert negotiator.negotiate('text/html') == 'rss'
```

### Cache Tests

```python
def test_feed_cache():
    """Test LRU cache with TTL"""
    cache = FeedCache(max_size=3, default_ttl=1)

    # Test set and get
    cache.set('rss', 50, 'abc123', '<rss>content</rss>', 'application/rss+xml')
    entry = cache.get('rss', 50, 'abc123')
    assert entry is not None
    assert entry.content == '<rss>content</rss>'

    # Test expiration
    time.sleep(1.1)
    entry = cache.get('rss', 50, 'abc123')
    assert entry is None

    # Test LRU eviction
    cache.set('rss', 50, 'aaa', 'content1', 'application/rss+xml')
    cache.set('atom', 50, 'bbb', 'content2', 'application/atom+xml')
    cache.set('json', 50, 'ccc', 'content3', 'application/json')
    cache.set('rss', 100, 'ddd', 'content4', 'application/rss+xml')  # Evicts oldest

    assert cache.get('rss', 50, 'aaa') is None  # Evicted
    assert cache.get('atom', 50, 'bbb') is not None  # Still present
```

### Statistics Tests

```python
def test_syndication_stats():
    """Test statistics collection"""
    stats = SyndicationStats()

    # Record requests
    stats.record_request('rss', 'Feedly/1.0', cached=False, generation_time=0.05)
    stats.record_request('atom', 'Inoreader/1.0', cached=True)
    stats.record_request('json', 'NetNewsWire/6.0', cached=False, generation_time=0.03)

    summary = stats.get_summary()
    assert summary['total_requests'] == 3
    assert 'rss' in summary['format_distribution']
    assert len(summary['top_user_agents']) > 0
```

### OPML Tests

```python
def test_opml_generation():
    """Test OPML export"""
    generator = OPMLGenerator(
        site_url='https://example.com',
        site_name='Test Site',
        owner_name='John Doe'
    )

    opml = generator.generate(['rss', 'atom', 'json'])

    # Parse and validate
    import xml.etree.ElementTree as ET
    root = ET.fromstring(opml)

    assert root.tag == 'opml'
    assert root.get('version') == '2.0'

    # Check outlines
    outlines = root.findall('.//outline')
    assert len(outlines) == 3
    assert outlines[0].get('type') == 'rss'
    assert outlines[1].get('type') == 'atom'
    assert outlines[2].get('type') == 'json'
```

## Performance Benchmarks

### Negotiation Performance

```python
def benchmark_content_negotiation():
    """Benchmark negotiation speed"""
    negotiator = ContentNegotiator()
    complex_header = 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8'

    start = time.perf_counter()
    for _ in range(10000):
        negotiator.negotiate(complex_header)
    duration = time.perf_counter() - start

    per_call = (duration / 10000) * 1000  # Convert to ms
    assert per_call < 1.0  # Less than 1ms per negotiation
```

## Configuration

```ini
# Content negotiation
STARPUNK_FEED_NEGOTIATION_ENABLED=true
STARPUNK_FEED_DEFAULT_FORMAT=rss

# Cache settings
STARPUNK_FEED_CACHE_ENABLED=true
STARPUNK_FEED_CACHE_SIZE=100
STARPUNK_FEED_CACHE_TTL=300
STARPUNK_FEED_CACHE_MEMORY_LIMIT=10  # MB

# Statistics
STARPUNK_FEED_STATS_ENABLED=true
STARPUNK_FEED_STATS_RETENTION=7  # days

# OPML
STARPUNK_FEED_OPML_ENABLED=true
STARPUNK_FEED_OPML_OWNER_NAME=
STARPUNK_FEED_OPML_OWNER_EMAIL=
```

## Security Considerations

1. **Cache Poisoning**: Validate all cached content
2. **Header Injection**: Sanitize Accept headers
3. **Memory Exhaustion**: Limit cache size
4. **Statistics Privacy**: Don't log sensitive data
5. **OPML Injection**: Escape all XML content

## Acceptance Criteria

1. ✅ Content negotiation working correctly
2. ✅ Cache hit rate >80% achieved
3. ✅ Statistics dashboard functional
4. ✅ OPML export valid
5. ✅ Memory usage bounded
6. ✅ Performance targets met
7. ✅ All formats properly cached
8. ✅ Invalidation working
9. ✅ User agent detection accurate
10. ✅ Security review passed