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/lib/email.ts

@@ -1,4 +1,5 @@
 import nodemailer from "nodemailer";
+import { mailQueue } from "../queues/index.js";
 
 const transporter = nodemailer.createTransport({
   host: process.env.SMTP_HOST || "localhost",
@@ -32,3 +33,13 @@ export async function sendPasswordReset(email: string, token: string): Promise<v
     html: `<p>Click <a href="${BASE_URL}/password/reset?token=${token}">here</a> to reset your password.</p>`,
   });
 }
+
+const jobOpts = { attempts: 3, backoff: { type: "exponential" as const, delay: 5000 } };
+
+export async function enqueueVerification(email: string, token: string): Promise<void> {
+  await mailQueue.add("sendVerification", { email, token }, jobOpts);
+}
+
+export async function enqueuePasswordReset(email: string, token: string): Promise<void> {
+  await mailQueue.add("sendPasswordReset", { email, token }, jobOpts);
+}