feat: migrate persistence from JSON to SQLModel (Phase 1)

- Integrated SQLModel with SQLite for robust data persistence
- Refactored UserManager and WatchlistManager to use SQL queries
- Migrated models to SQLModel with relationships and primary keys
- Updated test suite with in-memory database isolation
- Removed deprecated JSON storage files

main.py

@@ -52,6 +52,11 @@ episode_checker.set_download_manager(download_manager)
 @app.on_event("startup")
 async def startup_event():
     """Initialize services on application startup"""
+    # Create database tables if they don't exist
+    from app.database import create_db_and_tables
+    create_db_and_tables()
+    logger.info("Database tables initialized")
+
     from app.sonarr_handler import get_sonarr_handler
 
     sonarr_handler = get_sonarr_handler()