Replace in-memory MessageLog with SQLite-backed implementation.
Same API surface (append/all/clear/len) so zero caller changes needed.
- data/chat.db stores messages with role, content, timestamp, source
- Lazy DB connection (opened on first use, not at import time)
- Retention policy: oldest messages pruned when count > 500
- New .recent(limit) method for efficient last-N queries
- Thread-safe with explicit locking
- WAL mode for concurrent read performance
- Test isolation: conftest redirects DB to tmp_path per test
- 8 new tests: persistence, retention, concurrency, source field
Closes#46
