Fix NPM forwarding: use HOST_IP env var instead of socket detection

Socket detection inside Docker returns the container IP (172.18.0.x),
not the host IP. Now:
- install.sh detects host IP via hostname -I and stores in .env
- docker-compose.yml passes HOST_IP to the container
- npm_service.py reads HOST_IP from environment
- Increased SSL cert timeout to 120s (LE validation is slow)
- Added better logging for SSL cert creation/assignment
- README updated with HOST_IP in .env example

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

README.md

@@ -257,6 +257,7 @@ DATA_DIR=/opt/netbird-instances
 DOCKER_NETWORK=npm-network
 LOG_LEVEL=INFO
 WEB_UI_PORT=8000
+HOST_IP=<your-server-ip>
 ```
 
 > **All application settings** (domain, NPM credentials, Docker images, branding, etc.) are stored in the SQLite database and editable via the Web UI under **Settings**.