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>

package.json

@@ -13,12 +13,12 @@
   },
   "dependencies": {
     "@hono/node-server": "^1.13.8",
+    "bullmq": "^5.34.8",
     "hono": "^4.7.6",
-    "node-cron": "^3.0.3"
+    "ioredis": "^5.6.1"
   },
   "devDependencies": {
     "@types/node": "^22.15.3",
-    "@types/node-cron": "^3.0.11",
     "tsx": "^4.19.4",
     "typescript": "^5.8.3",
     "eslint": "^9.25.1",