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/main.py

@@ -7,11 +7,11 @@ from fastapi import FastAPI, Request
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse
 from fastapi.staticfiles import StaticFiles
-from slowapi import Limiter, _rate_limit_exceeded_handler
+from slowapi import _rate_limit_exceeded_handler
 from slowapi.errors import RateLimitExceeded
-from slowapi.util import get_remote_address
 
 from app.database import init_db
+from app.limiter import limiter
 from app.routers import auth, customers, deployments, monitoring, settings, users
 
 # ---------------------------------------------------------------------------
@@ -27,11 +27,6 @@ logger = logging.getLogger(__name__)
 # ---------------------------------------------------------------------------
 # Application
 # ---------------------------------------------------------------------------
-# ---------------------------------------------------------------------------
-# Rate limiter (SlowAPI)
-# ---------------------------------------------------------------------------
-limiter = Limiter(key_func=get_remote_address)
-
 # ---------------------------------------------------------------------------
 # Application
 # ---------------------------------------------------------------------------