Phase 2: Job enqueueing from counter

- Add backend/jobs.py with enqueue_random_number_job function
- Modify counter increment endpoint to enqueue job after incrementing
- Add mock_enqueue_job fixture to conftest.py for all tests
- Add test_increment_enqueues_job_with_user_id to verify correct user_id
- Job is enqueued synchronously; failure causes request to fail

backend/jobs.py

@@ -0,0 +1,39 @@
+"""Job definitions and enqueueing utilities using pgqueuer."""
+
+import json
+import os
+
+import asyncpg
+from pgqueuer.queries import Queries
+
+# Job type constants
+JOB_RANDOM_NUMBER = "random_number"
+
+# SQLAlchemy uses postgresql+asyncpg://, but asyncpg needs postgresql://
+_raw_url = os.getenv(
+    "DATABASE_URL", "postgresql+asyncpg://postgres:postgres@localhost:5432/arbret"
+)
+DATABASE_URL = _raw_url.replace("postgresql+asyncpg://", "postgresql://")
+
+
+async def enqueue_random_number_job(user_id: int) -> int:
+    """
+    Enqueue a random number job for the given user.
+
+    Args:
+        user_id: The ID of the user who triggered the job.
+
+    Returns:
+        The job ID.
+
+    Raises:
+        Exception: If enqueueing fails.
+    """
+    conn = await asyncpg.connect(DATABASE_URL)
+    try:
+        queries = Queries.from_asyncpg_connection(conn)
+        payload = json.dumps({"user_id": user_id}).encode()
+        job_ids = await queries.enqueue(JOB_RANDOM_NUMBER, payload)
+        return job_ids[0]
+    finally:
+        await conn.close()