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/feeds/__init__.py

@@ -13,6 +13,9 @@ Exports:
     generate_json_feed_streaming: Generate JSON Feed 1.1 with streaming
     negotiate_feed_format: Content negotiation for feed formats
     get_mime_type: Get MIME type for a format name
+    get_cache: Get global feed cache instance
+    configure_cache: Configure global feed cache
+    FeedCache: Feed caching class
 """
 
 from .rss import (
@@ -38,6 +41,12 @@ from .negotiation import (
     get_mime_type,
 )
 
+from .cache import (
+    FeedCache,
+    get_cache,
+    configure_cache,
+)
+
 __all__ = [
     # RSS functions
     "generate_rss",
@@ -54,4 +63,8 @@ __all__ = [
     # Content negotiation
     "negotiate_feed_format",
     "get_mime_type",
+    # Caching
+    "FeedCache",
+    "get_cache",
+    "configure_cache",
 ]