feat: add notifications module with bell UI, event listeners, and cleanup job

In-app notification system with topbar bell icon, dropdown panel,
mark-as-read/dismiss actions, and scheduled cleanup of old read
notifications. Includes event listeners for user.created and
user.deleted events, authorization policy, and full test coverage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/Service/User/UserAccountService.php

@@ -102,6 +102,12 @@ class UserAccountService
             ];
         }
 
+        // Capture tenant associations before CASCADE deletes them
+        $preDeletionTenantIds = array_column(
+            $this->userAssignmentService->buildAssignmentsForUser($userId)['tenants'] ?? [],
+            'id'
+        );
+
         $deleted = $this->userWriteRepository->delete($userId);
         if (!$deleted) {
             return ['ok' => false, 'status' => 500, 'error' => 'delete_failed'];
@@ -118,6 +124,8 @@ class UserAccountService
             'user_id' => $userId,
             'uuid' => (string) ($user['uuid'] ?? ''),
             'actor_user_id' => $currentUserId,
+            'display_name' => trim((string) ($user['display_name'] ?? '')),
+            'tenant_ids' => $preDeletionTenantIds,
         ]);
 
         return ['ok' => true, 'user' => $user];