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/queues/+page.server.ts

@@ -0,0 +1,7 @@
+import { getAdminQueues } from "$lib/services";
+
+export async function load({ fetch, cookies }) {
+  const token = cookies.get("session_token") || "";
+  const queues = await getAdminQueues(fetch, token).catch(() => []);
+  return { queues };
+}