feat(auto-update): per-stack auto-update enable/disable toggle (#771)

* feat(auto-update): add per-stack auto-update enable/disable toggle

Paid users (Skipper and Admiral) can now opt individual stacks out of
scheduled auto-updates from the stack context menu without disabling
the global feature.

- Add stack_auto_update_settings table (node_id, stack_name) with
  default enabled=true; four typed DatabaseService accessors with
  parameterized queries.
- Add GET /stacks/auto-update-settings, GET /stacks/:name/auto-update,
  and PUT /stacks/:name/auto-update (requirePaid + requireAdmin).
  PUT broadcasts state-invalidate with action auto-update-settings-changed
  so all open tabs refresh immediately.
- Stack DELETE clears the auto-update setting row alongside stack_update_status.
- autoUpdateRouter /execute skips disabled stacks before any registry
  call; skip is recorded in the results array. Manual Update actions
  are not affected.
- Add Auto-update: Enabled/Disabled toggle in the stack inspect group
  (paid tiers only, hidden for Community, consistent with Auto-Heal).
  Toggle uses optimistic update with revert-on-error toast.
- AutoUpdateReadinessView shows an Auto: Off pill and disables the
  Apply now button for stacks with auto-updates off. Detection still
  runs so the readiness card remains visible.
- Add 21 backend Vitest tests covering DB round-trips, endpoint auth
  and tier gates, execute skip for both wildcard and named targets.
  Add 3 frontend hook tests for toggle visibility and callback behavior.

* docs(auto-update): document per-stack auto-update control

Add a Per-stack control section to the auto-update readiness page
explaining how to disable and re-enable auto-updates for individual
stacks, what disabling means (scheduled apply skipped; detection still
runs; manual update unaffected), and a troubleshooting entry for
scheduled runs not applying to a specific stack.

frontend/src/components/sidebar/sidebar-types.ts

@@ -26,11 +26,16 @@ export interface StackMenuCtx {
   hasPort: boolean;
   isBusy: boolean;
   isPaid: boolean;
+  // isAdmiral: plumbed now so Admiral-specific menu items can be added in a
+  // follow-up PR without changing this interface. No Admiral-only items ship
+  // in the current PR; the auto-update toggle is Skipper+, gated on isPaid.
+  isAdmiral: boolean;
   canDelete: boolean;
   isPinned: boolean;
   labels: Label[];
   assignedLabelIds: number[];
   menuVisibility: { showDeploy: boolean; showStop: boolean; showRestart: boolean; showUpdate: boolean };
+  autoUpdateEnabled: boolean;
   openAlertSheet: () => void;
   openAutoHeal: () => void;
   checkUpdates: () => void;
@@ -45,6 +50,7 @@ export interface StackMenuCtx {
   toggleLabel: (labelId: number) => void;
   createAndAssignLabel: (name: string, color: LabelColor) => Promise<void>;
   openLabelManager: () => void;
+  setAutoUpdateEnabled: (enabled: boolean) => void;
 }
 
 export type StackGroupKind = 'pinned' | 'labeled' | 'unlabeled';