add: Optional router-level API key that gates router/API/web UI access

Optional router-level API key that gates router/API/web UI access (leave empty to disable)

## Supplying the router API key

If you set `nomyo-router-api-key` in `config.yaml` (or `NOMYO_ROUTER_API_KEY` env), every request to NOMYO Router must include the key:

- HTTP header (recommended): `Authorization: Bearer <router_key>`
- Query param (fallback): `?api_key=<router_key>`

Examples:
```bash
curl -H "Authorization: Bearer $NOMYO_ROUTER_API_KEY" http://localhost:12434/api/tags
curl "http://localhost:12434/api/tags?api_key=$NOMYO_ROUTER_API_KEY"
```

doc/examples/sample-config.yaml

@@ -6,6 +6,9 @@ endpoints:
 
 max_concurrent_connections: 2
 
+# Optional router-level API key to secure the router and dashboard (leave blank to disable)
+nomyo-router-api-key: ""
+
 # Multi-endpoint configuration with local Ollama instances
 # endpoints:
 #   - http://ollama-worker1:11434