Add GameStateManager service with Redis/Postgres dual storage
- Implement GameStateManager with Redis-primary, Postgres-backup pattern
- Cache operations: save_to_cache, load_from_cache, delete_from_cache
- DB operations: persist_to_db, load_from_db, delete_from_db
- High-level: load_state (cache-first), delete_game, recover_active_games
- Query helpers: get_active_game_count, get_player_active_games
- Add 22 tests for GameStateManager (87% coverage)
- Add 6 __repr__ tests for all DB models (100% model coverage)
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