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/workers/mail.ts

@@ -0,0 +1,28 @@
+import { Worker } from "bullmq";
+import { redisConnectionOpts } from "../connection.js";
+import { sendVerification, sendPasswordReset } from "../../lib/email.js";
+
+export function startMailWorker(): Worker {
+  const worker = new Worker(
+    "mail",
+    async (job) => {
+      switch (job.name) {
+        case "sendVerification":
+          await sendVerification(job.data.email as string, job.data.token as string);
+          break;
+        case "sendPasswordReset":
+          await sendPasswordReset(job.data.email as string, job.data.token as string);
+          break;
+        default:
+          throw new Error(`Unknown mail job: ${job.name}`);
+      }
+    },
+    { connection: redisConnectionOpts },
+  );
+
+  worker.on("failed", (job, err) => {
+    console.error(`Mail job ${job?.id} (${job?.name}) failed:`, err.message);
+  });
+
+  return worker;
+}