0.36.0: per-stack image-update indicator on the Stacks overview

Shows an amber "Update" pill next to a stack's status (and highlights the
inline Update button) when any of the stack's images has a newer digest in
the registry. Reuses the existing background image-update check — a new
update_service.stacks_update_summary() reads the cached digests in a single
container sweep (no extra registry calls), exposed as GET /api/stacks/updates
and proxied per agent at GET /api/agents/{id}/stacks/updates. The Stacks page
and each remote-host section poll it every 60s.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/agent_app.py

@@ -280,6 +280,12 @@ def stacks_stats() -> dict:
     return stats_service.stack_stats()
 
 
+@app.get("/agent/stacks/updates", dependencies=[Depends(verify_token)])
+def stacks_updates() -> dict:
+    """Per-stack image-update availability from the cached digests."""
+    return update_service.stacks_update_summary()
+
+
 @app.get("/agent/stacks/{stack_id}", dependencies=[Depends(verify_token)])
 def get_stack(stack_id: str) -> dict:
     directory = compose_service.stack_dir(stack_id)