fix(api): document dispose_engine lazy import + regression test (CAR-1135)

- main.py: add docstring inside the lifespan function explaining why
  dispose_engine is lazy-imported rather than top-level. The original
  import path (top-level) crashed the container at import time with
  'ImportError: cannot import name dispose_engine from cartsnitch_api.database'
  when database.py was stale or stripped during a CI build. Lazy import
  keeps the engine disposal behavior while preventing the module-load
  crash.
- tests/test_openapi.py: add test_dispose_engine_importable_from_database
  that asserts dispose_engine is importable and callable. This is the
  exact path the deployed UAT image was failing on, captured as a
  regression test so a future regression lands in CI before deploy.

Refs CAR-1135.

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

src/cartsnitch_api/main.py

@@ -25,6 +25,12 @@ from cartsnitch_api.routes.user import router as user_router
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
+    # Lazy import: keep `dispose_engine` out of the top-level imports so a
+    # stale or partially-built database.py never breaks module load on
+    # container start. The function is required for graceful pool cleanup
+    # on shutdown; if the import fails, the cache_client.close() that
+    # follows the yield would mask it. See CAR-1135 for the original
+    # ImportError that motivated this pattern.
     from cartsnitch_api.database import dispose_engine
 
     await cache_client.initialize()