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

Docker/compose.prod.yml

@@ -48,6 +48,7 @@ services:
       target: runtime
     container_name: bangui-backend
     restart: unless-stopped
+    stop_grace_period: 30s  # Give lifespan 30s to complete before SIGKILL
     depends_on:
       fail2ban:
         condition: service_healthy