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/graphql/index.ts

@@ -9,6 +9,7 @@ import "./resolvers/recordings.js";
 import "./resolvers/comments.js";
 import "./resolvers/gamification.js";
 import "./resolvers/stats.js";
+import "./resolvers/queues.js";
 import { builder } from "./builder";
 
 export const schema = builder.toSchema();