Migrate task queue from in-memory to BullMQ + Redis

Replace the volatile in-memory Map/Set queue with BullMQ backed by Redis
for persistence across restarts, automatic retry with exponential backoff,
and non-blocking workspace cleanup.

- Two queues: workspace-create (concurrency-limited) and workspace-cleanup
  (with polling instead of sleep-based stop→delete)
- Active workspaces tracked in Redis hash for dedup across restarts
- Stale sweep every 10 minutes catches orphaned workspaces
- Graceful shutdown on SIGTERM/SIGINT
- Replace node-cron with BullMQ repeatable jobs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/handlers/issue-comment.ts

@@ -43,7 +43,7 @@ export function issueComment(config: Config, queue: TaskQueue) {
       giteaToken: config.giteaDevToken,
     };
 
-    const queued = queue.enqueue(task);
+    const queued = await queue.enqueue(task);
 
     return c.json({ ok: true, queued });
   };