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.

backend/src/routes/imageUpdates.ts

@@ -219,6 +219,11 @@ autoUpdateRouter.post('/execute', authMiddleware, async (req: Request, res: Resp
 
     for (const stackName of stackNames) {
       try {
+        if (!db.getStackAutoUpdateEnabled(req.nodeId, stackName)) {
+          results.push(`Stack "${stackName}": auto-updates disabled; skipped.`);
+          continue;
+        }
+
         const containers = await docker.getContainersByStack(stackName);
         if (!containers || containers.length === 0) {
           results.push(`Stack "${stackName}": no containers found; skipped.`);