Phase 22: auto-update (Watchtower-style), local + agent (0.28.0)

Per-stack auto-update policy on the stack Overview tab. When the background
image-update check finds a newer registry digest for one of a stack's images,
the stack is pulled + redeployed (or just flagged, "notify only"). Only running
stacks are auto-redeployed; a stopped stack is skipped, never silently started.

- models/auto_update.py: AutoUpdate(stack_id, agent_id, enabled, redeploy,
  last_run/status/result) + schemas; registered in models/__init__.py.
- update_service: DB-free stack_images/stack_updates helpers (agent reuses
  them); agent GET /agent/stacks/{id}/updates.
- services/auto_update_service.py: run_due/run_policy (local pull+up via
  compose_service, remote via agent_service POST /agent/stacks/{id}/update,
  notify-only with per-transition dedup); lazy-called from
  update_service.background_loop. New stack_auto_updated notify event.
- routers: GET/PUT/run /api/stacks/{id}/auto-update and the
  /api/agents/{id}/stacks/{sid}/auto-update variants (policy stored centrally).
- frontend: api/autoUpdate.ts + AutoUpdatePanel (enable, redeploy|notify-only,
  Check now, last-run status) on StackDetail + RemoteStackDetail; EVENT_LABELS
  gains stack_auto_updated + backup_failed.

Live-verified all four paths (updated / update-available / up-to-date /
skipped) against real compose.

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

frontend/src/pages/StackDetail.tsx

@@ -16,6 +16,7 @@ import { Badge, Button, Card, Spinner, StatusDot } from "@/components/ui";
 import { ConfirmDialog } from "@/components/ui/ConfirmDialog";
 import { LogViewer } from "@/components/stacks/LogViewer";
 import { ContainerCard } from "@/components/stacks/ContainerCard";
+import { AutoUpdatePanel } from "@/components/stacks/AutoUpdatePanel";
 import { BackupButton } from "@/components/stacks/BackupRestore";
 import { stacksApi } from "@/api/stacks";
 import { apiErrorMessage } from "@/api/client";
@@ -130,6 +131,7 @@ function Overview({
 }) {
   return (
     <div className="space-y-2 overflow-auto">
+      <AutoUpdatePanel stackId={data.id} isAdmin={isAdmin} />
       {data.containers.length === 0 && (
         <Card>
           <p className="text-sm text-slate-500">