feat(backend): implement postgres job queue

backend/README.md

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+# Backend
+
+Django REST Framework backend for the minimal queue.
+
+The backend owns two models:
+
+- `Job`
+- `JobEvent`
+
+Workers do not call public claim endpoints. The worker command imports the same service functions that API views use for creation, retry, stats, and event reads.
+
+## API
+
+```http
+GET  /api/health/
+POST /api/jobs/
+GET  /api/jobs/
+GET  /api/jobs/{id}/
+GET  /api/jobs/{id}/events/
+GET  /api/job-events/?after_id=123&limit=100
+GET  /api/jobs/stats/
+POST /api/jobs/{id}/retry/
+```
+
+## Worker
+
+```powershell
+python manage.py run_job_workers
+```
+
+Worker settings come from environment variables:
+
+```env
+JOB_WORKER_THREADS=4
+JOB_WORKER_POLL_INTERVAL_MS=500
+JOB_WORKER_LEASE_SECONDS=30
+JOB_WORKER_CLEANUP_INTERVAL_SECONDS=5
+JOB_WORKER_BACKOFF_BASE_SECONDS=5
+JOB_WORKER_BACKOFF_MAX_SECONDS=300
+JOB_WORKER_MAX_ATTEMPTS_DEFAULT=3
+```