feat(project-pause): set/clear paused_at on toggle and bulk pause-resume

app/app/Http/Controllers/Api/ProjectController.php

@@ -164,7 +164,14 @@ class ProjectController extends Controller
     {
         $request->validate(['is_active' => ['required', 'boolean']]);
         $project = Project::where('tenant_id', $request->user()->tenant_id)->findOrFail($id);
-        $project->update(['is_active' => $request->boolean('is_active')]);
+
+        // Spec: docs/superpowers/plans/2026-05-26-supplier-snapshot-guard.md (Task 11).
+        // paused_at — anchor для SupplierSnapshotGuard grace-расчёта.
+        $newActive = $request->boolean('is_active');
+        $project->update([
+            'is_active' => $newActive,
+            'paused_at' => $newActive ? null : now(),
+        ]);
 
         // #10: pause/resume must reach the supplier. The job's group recompute pushes
         // status=paused when no active project of the group remains (resume → active).