chore: round-4 polish + standardize on module-level loggers

Round-4 review (non-blocking) items:
- get_procrastinate_conninfo: warn on an empty connect_timeout= value (it falls
  back to the 10s default); preserve an explicit connect_timeout=0.
- Document the _doc_queueing_lock user_id invariant (NC rejects ':' in usernames).
- docs/configuration.md: note that `db downgrade` leaves procrastinate's tables
  in place and how to drop them on a full teardown.
- reclaim_stalled_ingest_jobs: debug heartbeat log when nothing is stalled.
- Drop the redundant list() wrap in the integration stalled-jobs assertion.

Logging pattern: define a module-level `logger = logging.getLogger(__name__)`
and use it instead of function-local or inline getLogger(__name__) calls
(config.py, config_validators.py, tests/.../test_scope_authorization.py). The
test file's dev-only `scripts.*` import gets a ty: ignore since it resolves via
sys.path at runtime, not as an installed package.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/integration/test_ingest_queue_postgres.py

@@ -133,9 +133,7 @@ async def test_ingest_queue_end_to_end(fresh_app):
         assert counts.get("todo") == 2
 
         # 4. Fresh todo jobs are not "doing", so none are stalled.
-        stalled = list(
-            await fresh_app.job_manager.get_stalled_jobs(
-                queue=INGEST_QUEUE_NAME, seconds_since_heartbeat=0
-            )
+        stalled = await fresh_app.job_manager.get_stalled_jobs(
+            queue=INGEST_QUEUE_NAME, seconds_since_heartbeat=0
         )
-        assert stalled == []
+        assert list(stalled) == []