- Replace unbounded Set with Map<sessionId, timestamp> tracking insertion time
- Add periodic cleanup every hour to remove entries older than 24h
- Add on-demand cleanup before duplicate checks for timely cleanup
- Add comprehensive TDD tests verifying TTL behavior:
* Fresh entries work correctly
* Stale entries (>24h) get cleaned up
* Fresh entries survive cleanup
* Bounded size with many entries
- All 447 tests pass including 4 new TTL tests
- Memory leak fixed while preserving DB-level deduplication
