fix: scope headless-shell to a dedicated network, off the Traefik proxy network

Prod override put headless-shell on the externally-managed `proxy`
network so manga-api (confined there for Traefik routing) could still
resolve it. That reopened CDP (port 9222, raw remote code execution)
to every other container on that shared network, not just manga-api.

Give both services a project-private `browser` network (defined in
the base compose file, not `internal: true` since headless Chrome
needs outbound access to kagane.to). manga-api joins both `proxy` and
`browser` in the prod override; headless-shell never touches `proxy`.

docker-compose.yml

@@ -45,6 +45,8 @@ services:
     # the public internet does not.
     ports:
       - "127.0.0.1:8080:8080"
+    networks:
+      - browser
 
   headless-shell:
     image: chromedp/headless-shell:stable
@@ -55,12 +57,20 @@ services:
     # container's lifetime.
     init: true
     # Deliberately no `ports:` — an exposed CDP endpoint is remote code
-    # execution. Only the backend on the internal network may reach it.
+    # execution. Only manga-api, via the `browser` network below, may reach it.
     command:
       - --remote-debugging-address=0.0.0.0
       - --remote-debugging-port=9222
       - --disable-gpu
       - --no-sandbox
+    networks:
+      - browser
 
 volumes:
   bookmarks-data:
+
+networks:
+  # Not `internal: true`: headless Chrome still needs outbound access to reach
+  # kagane.to. Isolation here comes from membership (only manga-api and
+  # headless-shell join it), not from cutting egress.
+  browser: