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>

nextcloud_mcp_server/vector/queue/procrastinate.py

@@ -136,6 +136,10 @@ async def reclaim_stalled_ingest_jobs(context: JobContext, timestamp: int) -> No
         reclaimed += 1
     if reclaimed:
         logger.warning("ingest.reclaimed_stalled_jobs count=%d", reclaimed)
+    else:
+        # Visible heartbeat under verbose logging when debugging a suspected
+        # reclaim failure, without noising up production logs.
+        logger.debug("ingest.reclaim_check stalled=0")
 
 
 async def _resolve_client(user_id: str) -> NextcloudClient:
@@ -293,11 +297,16 @@ def _doc_queueing_lock(task: DocumentTask) -> str:
     """Per-document enqueue-dedup key (partial-unique on ``status='todo'``).
 
     Collision-safe with a raw ``:`` delimiter because the first two segments can
-    never themselves contain ``:``: ``user_id`` is a Nextcloud username/UID (no
-    colons) and ``doc_type`` is a controlled enum (``note``/``file``/
-    ``deck_card``/``news_item``). The trailing ``doc_id`` may contain anything —
-    it's the final unambiguous segment. A future ``doc_type`` containing ``:``
-    would break this invariant, so keep doc_type colon-free.
+    never themselves contain ``:``:
+
+    - ``user_id`` — a Nextcloud username/UID; Nextcloud rejects ``:`` in
+      usernames (Web UI + provisioning validation), so it is colon-free.
+    - ``doc_type`` — a controlled enum (``note``/``file``/``deck_card``/
+      ``news_item``); a future ``doc_type`` containing ``:`` would break this
+      invariant, so keep doc_type colon-free.
+
+    The trailing ``doc_id`` may contain anything — it's the final unambiguous
+    segment.
     """
     return f"{task.user_id}:{task.doc_type}:{task.doc_id}"