feat: add BullMQ job queue with admin monitoring UI

- Add BullMQ to backend; mail jobs (verification, password reset) now enqueued instead of sent inline
- Mail worker processes jobs with 3-attempt exponential backoff retry
- Admin GraphQL resolvers: adminQueues, adminQueueJobs, adminRetryJob, adminRemoveJob, adminPauseQueue, adminResumeQueue
- Admin frontend page at /admin/queues: queue cards with counts, job table with status filter, retry/remove/pause actions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/routes/admin/+layout.svelte

@@ -14,6 +14,11 @@
       href: "/admin/recordings",
       icon: "icon-[ri--record-circle-line]",
     },
+    {
+      name: $_("admin.nav.queues"),
+      href: "/admin/queues",
+      icon: "icon-[ri--stack-line]",
+    },
   ]);
 
   function isActive(href: string) {