feat(backend): implement postgres job queue

backend/.env.sample

@@ -0,0 +1,17 @@
+DEBUG=true
+DJANGO_SECRET_KEY=change-me
+DJANGO_ALLOWED_HOSTS=localhost,127.0.0.1,0.0.0.0
+DJANGO_CORS_ALLOWED_ORIGINS=http://localhost:5173,http://127.0.0.1:5173
+POSTGRES_DB=job_queue
+POSTGRES_USER=postgres
+POSTGRES_PASSWORD=postgres
+POSTGRES_HOST=localhost
+POSTGRES_PORT=5432
+JOB_WORKER_THREADS=4
+JOB_WORKER_POLL_INTERVAL_MS=500
+JOB_WORKER_LEASE_SECONDS=30
+JOB_WORKER_CLEANUP_INTERVAL_SECONDS=5
+JOB_WORKER_CLEANUP_BATCH_SIZE=100
+JOB_WORKER_BACKOFF_BASE_SECONDS=5
+JOB_WORKER_BACKOFF_MAX_SECONDS=300
+JOB_WORKER_MAX_ATTEMPTS_DEFAULT=3