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/connection.ts

@@ -0,0 +1,16 @@
+function parseRedisUrl(url: string): { host: string; port: number; password?: string } {
+  const parsed = new URL(url);
+  return {
+    host: parsed.hostname,
+    port: parseInt(parsed.port) || 6379,
+    password: parsed.password || undefined,
+  };
+}
+
+// BullMQ creates its own IORedis connections from these options.
+// maxRetriesPerRequest: null is required for workers.
+export const redisConnectionOpts = {
+  ...parseRedisUrl(process.env.REDIS_URL || "redis://localhost:6379"),
+  maxRetriesPerRequest: null as null,
+  enableReadyCheck: false,
+};