fix(headroom): support Docker sidecar proxy

Treat configured Headroom proxy as running when its /health endpoint
responds, even if local headroom CLI is not installed. Dashboard
Start/Stop stays limited to local loopback proxies while external
Docker sidecars can be enabled via HEADROOM_URL.

Closes #1948

Co-authored-by: Cursor <cursoragent@cursor.com>

DOCKER.md

@@ -64,6 +64,34 @@ docker run -d \
   decolua/9router:latest
 ```
 
+## Optional Headroom sidecar
+
+The 9Router image does not bundle Python or Headroom. To use Headroom in Docker, run it as a separate service and point 9Router at that proxy:
+
+```yaml
+services:
+  9router:
+    image: decolua/9router:latest
+    ports:
+      - "20128:20128"
+    volumes:
+      - "$HOME/.9router:/app/data"
+    environment:
+      DATA_DIR: /app/data
+      HEADROOM_URL: http://headroom:8787
+    depends_on:
+      - headroom
+
+  headroom:
+    image: ghcr.io/chopratejas/headroom:latest
+    ports:
+      - "8787:8787"
+```
+
+In the dashboard, open `Endpoint` → `Token Saver` → `Headroom`, confirm the URL is `http://headroom:8787`, recheck status, then enable Headroom.
+
+If Headroom runs on the Docker host instead of as a sidecar, use `http://host.docker.internal:8787` on macOS/Windows. On Linux, add `--add-host=host.docker.internal:host-gateway` or the equivalent compose `extra_hosts` entry.
+
 ## Update to latest
 
 ```bash