refactor(fc2c-i): single shared async engine with lifecycle dispose

backend/app/__init__.py

@@ -21,4 +21,9 @@ def create_app() -> Quart:
     # Registered last so /api/* routes win over the SPA catch-all.
     app.register_blueprint(frontend_bp)
 
+    @app.after_serving
+    async def _dispose_db_engine() -> None:
+        from .extensions import dispose_engine
+        await dispose_engine()
+
     return app