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/index.ts

@@ -16,6 +16,7 @@ import { db } from "./db/connection";
 import { redis } from "./lib/auth";
 import { logger } from "./lib/logger";
 import { migrate } from "drizzle-orm/node-postgres/migrator";
+import { startMailWorker } from "./queues/workers/mail";
 
 const PORT = parseInt(process.env.PORT || "4000");
 const UPLOAD_DIR = process.env.UPLOAD_DIR || "/data/uploads";
@@ -28,6 +29,10 @@ async function main() {
   await migrate(db, { migrationsFolder });
   logger.info("Migrations complete");
 
+  // Start background workers
+  startMailWorker();
+  logger.info("Queue workers started");
+
   const fastify = Fastify({ loggerInstance: logger });
 
   await fastify.register(fastifyCookie, {