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/routers/agents.py

@@ -226,6 +226,16 @@ async def agent_stacks_stats(
     return await _proxy(session, agent, "GET", "/agent/stacks/stats")
 
 
+@router.get("/{agent_id}/stacks/updates")
+async def agent_stacks_updates(
+    agent_id: int,
+    session: Session = Depends(get_session),
+    _user: User = Depends(get_current_user),
+) -> dict:
+    agent = _get_or_404(session, agent_id)
+    return await _proxy(session, agent, "GET", "/agent/stacks/updates")
+
+
 @router.get("/{agent_id}/stacks/{stack_id}")
 async def agent_stack_detail(
     agent_id: int,