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/database.py

@@ -51,6 +51,22 @@ def init_db() -> None:
     Base.metadata.create_all(bind=engine)
     _run_migrations()
 
+    # Insert default SystemConfig row (id=1) if it doesn't exist yet
+    db = SessionLocal()
+    try:
+        if not db.query(SystemConfig).filter(SystemConfig.id == 1).first():
+            db.add(SystemConfig(
+                id=1,
+                base_domain="example.com",
+                admin_email="admin@example.com",
+                npm_api_url="http://localhost:81",
+                npm_api_email_encrypted="",
+                npm_api_password_encrypted="",
+            ))
+            db.commit()
+    finally:
+        db.close()
+
 
 def _run_migrations() -> None:
     """Add columns that may be missing from older database versions."""