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

artifacts