fix: move dispose_engine import inside lifespan function

The top-level import of dispose_engine from cartsnitch_api.database
caused ImportError at module scope because dispose_engine was not
available when main.py was loaded. The function is only used inside
the lifespan context manager, so move the import there.

Fixes: ImportError cannot import name 'dispose_engine' from 'cartsnitch_api.database'

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/main.py

@@ -6,7 +6,6 @@ from fastapi import APIRouter, FastAPI
 
 from cartsnitch_api.auth.routes import router as auth_router
 from cartsnitch_api.cache import cache_client
-from cartsnitch_api.database import dispose_engine
 from cartsnitch_api.middleware.cors import add_cors_middleware
 from cartsnitch_api.middleware.error_handler import add_error_handlers, add_error_monitor_middleware
 from cartsnitch_api.middleware.rate_limit import add_rate_limit_middleware
@@ -26,6 +25,7 @@ from cartsnitch_api.routes.user import router as user_router
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
+    from cartsnitch_api.database import dispose_engine
     await cache_client.initialize()
     yield
     await cache_client.close()