Add Traefik reverse proxy support with automatic SSL

Configure Immich to work with existing Traefik instance:
- Add Traefik labels to immich-server for automatic routing
- Add external traefik network connection
- Disable direct port exposure (use Traefik instead)
- Add DOMAIN variable to .env.example
- Improve .gitignore for generated files and volumes
- Add comprehensive README with deployment instructions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.env.example

@@ -1,5 +1,8 @@
 # You can find documentation for all the supported env variables at https://docs.immich.app/install/environment-variables
 
+# Domain for Traefik reverse proxy
+DOMAIN=photos.example.com
+
 # The location where your uploaded files are stored
 UPLOAD_LOCATION=./library