feat: make ComfyUI backend IP/port configurable via environment variables

- Replace hardcoded IP in comfyui-nginx.conf with env vars
- Add COMFYUI_BACKEND_HOST and COMFYUI_BACKEND_PORT to compose.yaml
- Use envsubst to substitute variables at container startup
- Defaults: 100.121.199.88:8188 (current RunPod Tailscale IP)

ai/comfyui-nginx.conf

@@ -30,7 +30,7 @@ http {
 
         location / {
             # Proxy to ComfyUI on RunPod via Tailscale
-            proxy_pass http://100.121.199.88:8188;
+            proxy_pass http://${COMFYUI_BACKEND_HOST}:${COMFYUI_BACKEND_PORT};
 
             # WebSocket upgrade
             proxy_http_version 1.1;