feat: Implement Phase 3 Feed Caching (Partial)

Implements feed caching layer with LRU eviction, TTL expiration, and ETag support.

Phase 3.1: Feed Caching (Complete)
- LRU cache with configurable max_size (default: 50 feeds)
- TTL-based expiration (default: 300 seconds = 5 minutes)
- SHA-256 checksums for cache keys and ETags
- Weak ETag generation (W/"checksum")
- If-None-Match header support for 304 Not Modified responses
- Cache invalidation (全体 or per-format)
- Hit/miss/eviction statistics tracking
- Content-based cache keys (changes when notes are modified)

Implementation:
- Created starpunk/feeds/cache.py with FeedCache class
- Integrated caching into feed routes (RSS, ATOM, JSON Feed)
- Added ETag headers to all feed responses
- 304 Not Modified responses for conditional requests
- Configuration: FEED_CACHE_ENABLED, FEED_CACHE_MAX_SIZE
- Global cache instance with singleton pattern

Architecture:
- Two-level caching:
  1. Note list cache (simple dict, existing)
  2. Feed content cache (LRU with TTL, new)
- Cache keys include format + notes checksum
- Checksums based on note IDs + updated timestamps
- Non-streaming generators used for cacheable content

Testing:
- 25 comprehensive cache tests (100% passing)
- Tests for LRU eviction, TTL expiration, statistics
- Tests for checksum generation and consistency
- Tests for ETag generation and uniqueness
- All 114 feed tests passing (no regressions)

Quality Metrics:
- 114/114 tests passing (100%)
- Zero breaking changes
- Full backward compatibility
- Cache disabled mode supported (FEED_CACHE_ENABLED=false)

Performance Benefits:
- Database queries reduced (note list cached)
- Feed generation reduced (content cached)
- Bandwidth saved (304 responses)
- Memory efficient (LRU eviction)

Note: Phase 3 is partially complete. Still pending:
- Feed statistics dashboard
- OPML 2.0 export endpoint

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

starpunk/config.py

@@ -82,6 +82,10 @@ def load_config(app, config_override=None):
     app.config["FEED_MAX_ITEMS"] = int(os.getenv("FEED_MAX_ITEMS", "50"))
     app.config["FEED_CACHE_SECONDS"] = int(os.getenv("FEED_CACHE_SECONDS", "300"))
 
+    # Feed caching (v1.1.2 Phase 3)
+    app.config["FEED_CACHE_ENABLED"] = os.getenv("FEED_CACHE_ENABLED", "true").lower() == "true"
+    app.config["FEED_CACHE_MAX_SIZE"] = int(os.getenv("FEED_CACHE_MAX_SIZE", "50"))
+
     # Metrics configuration (v1.1.2 Phase 1)
     app.config["METRICS_ENABLED"] = os.getenv("METRICS_ENABLED", "true").lower() == "true"
     app.config["METRICS_SLOW_QUERY_THRESHOLD"] = float(os.getenv("METRICS_SLOW_QUERY_THRESHOLD", "1.0"))