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/config.ts

@@ -25,6 +25,14 @@ export interface Config {
   callbackUrl: string;
   /** Optional webhook secret for verifying Gitea signatures */
   webhookSecret?: string;
+  /** Redis connection URL (e.g. redis://redis:6379) */
+  redisUrl: string;
+  /** Max concurrent workspace creations (default: 2) */
+  queueConcurrency: number;
+  /** Max concurrent cleanup jobs (default: 5) */
+  cleanupConcurrency: number;
+  /** Minutes before a workspace is considered stale (default: 120) */
+  staleWorkspaceMinutes: number;
 }
 
 function required(name: string): string {
@@ -50,5 +58,9 @@ export function loadConfig(): Config {
     giteaUrl: process.env.GITEA_URL || "https://gitea.samson.media",
     callbackUrl: required("CALLBACK_URL"),
     webhookSecret: process.env.WEBHOOK_SECRET,
+    redisUrl: required("REDIS_URL"),
+    queueConcurrency: parseInt(process.env.QUEUE_CONCURRENCY || "2", 10),
+    cleanupConcurrency: parseInt(process.env.CLEANUP_CONCURRENCY || "5", 10),
+    staleWorkspaceMinutes: parseInt(process.env.STALE_WORKSPACE_MINUTES || "120", 10),
   };
 }