fix: resolve circular import, async blocking, SELinux and delete timeout issues

- Extract shared SlowAPI limiter to app/limiter.py to break circular
  import between app.main and app.routers.auth
- Seed default SystemConfig row (id=1) on first DB init so settings
  page works out of the box
- Make all docker_service.compose_* functions async (run_in_executor)
  so long docker pulls/stops no longer block the async event loop
- Propagate async to netbird_service stop/start/restart and await
  callers in deployments router
- Move customer delete to BackgroundTasks so the HTTP response returns
  immediately and avoids frontend "Network error" on slow machines
- docker-compose: add :z SELinux labels, mount docker.sock directly,
  add security_opt label:disable for socket access, extra_hosts for
  host.docker.internal, enable DELETE/VOLUMES on socket proxy
- npm_service: auto-detect outbound host IP via UDP socket when
  HOST_IP env var is not set

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/routers/auth.py

@@ -27,8 +27,7 @@ from app.utils.validators import ChangePasswordRequest, LoginRequest, MfaTokenRe
 logger = logging.getLogger(__name__)
 router = APIRouter()
 
-# Import the shared rate limiter from main
-from app.main import limiter
+from app.limiter import limiter
 
 
 @router.post("/login")