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/package.json

@@ -14,14 +14,15 @@
     "check": "tsc --noEmit"
   },
   "dependencies": {
-    "@sexy.pivoine.art/types": "workspace:*",
     "@fastify/cookie": "^11.0.2",
     "@fastify/cors": "^10.0.2",
     "@fastify/multipart": "^9.0.3",
     "@fastify/static": "^8.1.1",
     "@pothos/core": "^4.4.0",
     "@pothos/plugin-errors": "^4.2.0",
+    "@sexy.pivoine.art/types": "workspace:*",
     "argon2": "^0.43.0",
+    "bullmq": "^5.70.4",
     "drizzle-orm": "^0.44.1",
     "fastify": "^5.4.0",
     "fluent-ffmpeg": "^2.1.3",