Fix NPM forward host: use real host IP instead of Docker gateway

- npm_service._get_forward_host() now detects the actual host IP via
  UDP socket (works inside Docker containers) instead of using
  172.17.0.1 Docker gateway which NPM can't reach
- install.sh uses hostname -I for NPM forward host
- Removed npm_api_url parameter from _get_forward_host()

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

app/services/netbird_service.py

@@ -224,7 +224,7 @@ async def deploy_customer(db: Session, customer_id: int) -> dict[str, Any]:
         npm_proxy_id = None
         npm_stream_id = None
         if not local_mode:
-            forward_host = npm_service._get_forward_host(config.npm_api_url)
+            forward_host = npm_service._get_forward_host()
             npm_result = await npm_service.create_proxy_host(
                 api_url=config.npm_api_url,
                 npm_email=config.npm_api_email,