- Add playbooks table to src/db.py with UNIQUE(date, market) constraint
- PlaybookStore: save/load/delete, status management, match_count tracking,
list_recent with market filter, stats without full deserialization
- DayPlaybook JSON serialization via Pydantic model_dump_json/model_validate_json
- 23 tests, 100% coverage on playbook_store.py
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>