Complete Timeline and Discover feed implementation

This tag marks the complete implementation of the feed system including: - Timeline feed (user-specific, authenticated) - Discover feed (public, all communities) - Shared repository base (eliminated 700+ lines duplication) - HMAC-signed cursor pagination - Comprehensive integration tests - Aggregator system documentation To revert: git revert -m 1 feed-system-v1 To view changes: git show feed-system-v1