feat: graceful shutdown and WAL cleanup

- Add stop_grace_period to backend container for graceful shutdown
- Document WAL mode rationale and orphaned file cleanup in db.py
- Handle database close errors gracefully in lifespan
- Clean up orphaned WAL files during startup before opening DB
- Reorder imports and fix formatting in startup.py

backend/app/main.py

@@ -318,7 +318,12 @@ async def _lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
             log.error("scheduler_lock_release_failed", error=str(e))
 
         # 6. Close the database connection.
-        await startup_db.close()
+        try:
+            await startup_db.close()
+            log.debug("database_connection_closed")
+        except Exception as exc:
+            log.error("database_connection_close_failed", error=str(exc))
+
         log.info("bangui_shut_down")