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/backend/src/queues/index.ts

@@ -0,0 +1,8 @@
+import { Queue } from "bullmq";
+import { redisConnectionOpts } from "./connection.js";
+
+export const mailQueue = new Queue("mail", { connection: redisConnectionOpts });
+
+export const queues: Record<string, Queue> = {
+  mail: mailQueue,
+};