refactor(auto-update): retire per-stack gate, drive auto-update from schedules only (#1233)

* refactor(auto-update): retire per-stack gate, drive auto-update from schedules only

The per-stack Auto-update toggle in the stack sidebar context menu wrote a
gate row to `stack_auto_update_settings`, but actual updates only ran when a
`scheduled_tasks` row with `action='update'` fired. On a fresh install the
toggle was inert: detection ran every 6h, nothing was applied.

The same context menu already exposes `Schedule task`, which opens
ScheduledOperationsView pre-filled for the stack where the user can pick
`Auto-update Stack` and any cron. Keeping the toggle alongside that flow
duplicated the same action and turned the gate table into a parallel store
of "is a covering schedule active" derivable from `scheduled_tasks` itself.

Drop the gate model entirely:
- Backend: remove the `stack_auto_update_settings` table and its four
  accessors, the three routes under /api/stacks/*/auto-update, the per-stack
  skip in /api/auto-update/execute and SchedulerService.executeUpdate's
  fleet branch, and the clearStackAutoUpdateSetting call on stack delete.
  Dashboard `autoUpdate` count derives from scheduled_tasks (action='update'
  rows pinned to the node, total/enabled split).
- Frontend: drop the Auto-update entry from the sidebar context menu and its
  optimistic toggle plumbing. Drop autoUpdateSettings state, the
  /stacks/auto-update-settings fetch, and the auto-update-settings-changed
  WebSocket branch. Slim useSidebarActivitySummary (just nextRunAt; no
  enabled/total counts). AutoUpdateReadinessView's per-card autoUpdateEnabled
  now means "a covering enabled action='update' schedule exists" (per-stack
  row or fleet row on this node, earliest next_run_at wins, per-stack row
  wins on ties), with the gate-fetch removed.
- New: scheduledTasksRouter broadcasts scope: 'scheduled-tasks' on POST,
  PUT, PATCH /toggle, and DELETE so useConfigurationStatus and
  useNextAutoUpdateRun refetch under the 250ms debounce instead of waiting
  for the 60s poll. The broadcast is wrapped so a broken subscriber socket
  cannot turn a successful mutation into a 500.
- Docs: rewrite the "Per-stack control" section of auto-update-policies.mdx
  to describe the schedule-based model; update the matching troubleshooting
  entry. The misleading fleet-update help text in ScheduledOperationsView
  is corrected to reflect that every stack on the node is covered.

Tier parity: the surviving auto-update path (Schedule task -> Auto-update
Stack / All Stacks) is gated `requirePaid + requireAdmin` backend and
`isPaid + isAdmin` frontend, matching the gate the deleted routes carried.
The pre-commit grep returns no tier-related diff outside this PR's scope.

No data migration is provided: greenfield rules apply, and the leftover
table on already-shipped instances is harmless because no code reads or
writes it after this PR.

* docs: sweep remaining references to the per-stack auto-update toggle

The previous commit retired the per-stack Auto-update gate in favor of
configuring auto-update purely through scheduled tasks. This commit
removes the now-stale mentions of that toggle across the operator docs:

- docs/features/sidebar.mdx: drop the Auto-update entry from the Inspect
  group description, the matching screenshot alt-text, and the Skipper
  Note that listed it. Schedule task now carries the cross-link to
  Auto-Update Policies.
- docs/features/stack-management.mdx: drop the Auto-update list item;
  refresh the Schedule task entry to mention the Auto-update Stack action.
- docs/features/dashboard.mdx: rename the Configuration Status row from
  "Auto-update stacks" to "Auto-update schedules" with the new value
  shape, and rewrite the troubleshooting accordion to describe the
  scheduled-tasks invalidation path.
- docs/features/scheduled-operations.mdx: rewrite the Auto-update All
  Stacks row and helper text to reflect that every stack on the node is
  covered (no per-stack opt-out from this surface anymore).
- docs/features/multi-node.mdx: rewrite the Updates column definition to
  derive the Auto/Off flag from enabled Auto-update Stack / Auto-update
  All Stacks schedules instead of the removed per-stack policy.

The auto-update-policies.mdx rewrite in the previous commit already
covered the main reference page. The sidebar-context-menu.png screenshot
will be refreshed on release once the new menu is live in production;
the alt text is updated in this commit so it accurately describes the
shipping state.

No website edits needed: the Auto-Update Policies feature card description
("Schedule automatic image pulls and redeployments per stack on your own
cadence") and the feature matrix labels ("Auto-update stack schedule",
"Auto-update all stacks schedule") remain accurate under the new model.

* fix(stacks): drop orphaned requireAdmin import after auto-update route removal

CI's backend lint step flagged this PR's earlier deletion of the three
/api/stacks/*/auto-update routes: those handlers were the only callers of
`requireAdmin` inside routes/stacks.ts, leaving the named import on line 15
unreferenced. `requirePaid` and `effectiveTier` from the same line are still
in use elsewhere in the file and stay.

tsc --noEmit does not flag unused named imports; ESLint's no-unused-vars
does. Local backend lint reproduces and now reports 0 errors against the
existing 334-warning baseline.

backend/src/__tests__/dashboard-routes.test.ts

@@ -112,6 +112,77 @@ describe('GET /api/dashboard/configuration', () => {
     expect(res.body.security.scanPolicies.locked).toBe(false);
   });
 
+  it('counts autoUpdate as enabled action=update scheduled tasks targeting this node, not other actions', async () => {
+    const db = DatabaseService.getInstance();
+    const now = Date.now();
+    const nodeId = 1;
+    const baseTask = {
+      created_by: 'admin',
+      created_at: now,
+      updated_at: now,
+      last_run_at: null,
+      next_run_at: now + 3600_000,
+      last_status: null,
+      last_error: null,
+      prune_targets: null,
+      target_services: null,
+      prune_label_filter: null,
+    };
+    const idA = db.createScheduledTask({
+      ...baseTask,
+      name: 'au-on',
+      target_type: 'stack',
+      target_id: 'app1',
+      node_id: nodeId,
+      action: 'update',
+      cron_expression: '0 3 * * *',
+      enabled: 1,
+    });
+    const idB = db.createScheduledTask({
+      ...baseTask,
+      name: 'au-off',
+      target_type: 'stack',
+      target_id: 'app2',
+      node_id: nodeId,
+      action: 'update',
+      cron_expression: '0 3 * * *',
+      enabled: 0,
+    });
+    const idC = db.createScheduledTask({
+      ...baseTask,
+      name: 'scan-row',
+      target_type: 'system',
+      target_id: null,
+      node_id: nodeId,
+      action: 'scan',
+      cron_expression: '0 3 * * *',
+      enabled: 1,
+    });
+    const idD = db.createScheduledTask({
+      ...baseTask,
+      name: 'au-other-node',
+      target_type: 'stack',
+      target_id: 'app3',
+      node_id: 999,
+      action: 'update',
+      cron_expression: '0 3 * * *',
+      enabled: 1,
+    });
+    try {
+      const res = await request(app).get('/api/dashboard/configuration').set('Cookie', adminCookie);
+      expect(res.status).toBe(200);
+      // Two update rows on node 1 (one enabled, one disabled); the scan row
+      // and the update row on node 999 must not leak into the count.
+      expect(res.body.automation.autoUpdate.total).toBe(2);
+      expect(res.body.automation.autoUpdate.enabled).toBe(1);
+    } finally {
+      db.deleteScheduledTask(idA);
+      db.deleteScheduledTask(idB);
+      db.deleteScheduledTask(idC);
+      db.deleteScheduledTask(idD);
+    }
+  });
+
   it('does not leak agent URLs, tokens, or other secret material in the response', async () => {
     // Seed a node-1 agent with a Discord URL so the configuration path
     // exercises the `configured` truthy branch. The URL must never appear