readd queue-worker domain, add heartbewat to db-queue

server/src/domain/queue-workers/queue-worker.model.ts

@@ -0,0 +1,22 @@
+import { Schema, model } from "mongoose";
+import { QueueModes } from "@/domain/app-settings/app-settings.type.js";
+import { QueueWorkerDocument } from "@/domain/queue-workers/queue-worker.type.js";
+
+// Single source of truth for worker liveness: the TTL that GCs stale records and
+// the window readers use to count a worker as alive. Heartbeat cadence must stay well below it.
+export const WORKER_TTL_SECONDS = 180;
+
+const QueueWorkerSchema = new Schema<QueueWorkerDocument>(
+	{
+		_id: { type: String, required: true },
+		mode: { type: String, enum: QueueModes, required: true },
+		lastSeenAt: { type: Date, default: Date.now, expires: WORKER_TTL_SECONDS },
+	},
+	{ timestamps: true }
+);
+
+const QueueWorkerModel = model<QueueWorkerDocument>("QueueWorker", QueueWorkerSchema);
+
+export type { QueueWorkerDocument };
+export { QueueWorkerModel };
+export default QueueWorkerModel;