Clear the stack update pill immediately after a manual/auto update (0.38.4)

The amber image-update indicator is fed from update_service._CACHE, which
only the background loop refreshed — after a per-stack Update/Pull the
stale digests kept the pill on until the next pass. Now the local digests
are reconciled with the cached remote digests right after a successful
pull/update (local backend, agent lifecycle, auto-update pass), and the
frontend invalidates the stack-updates queries after actions.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/agent_app.py

@@ -374,6 +374,8 @@ async def lifecycle(stack_id: str, action: str) -> dict:
                 "detail": result.get("stderr", "").strip()[-2000:],
             },
         )
+    if action in ("pull", "update"):
+        update_service.refresh_stack_local(stack_id)
     return result