fix(ingest): address review round 2 (stale gauge + hygiene)

- metrics: update_ingest_queue_depth now pre-zeroes every managed ingest queue
  before applying live counts, so a queue that drains to empty (and drops out of
  procrastinate's list_queues_async) reads 0 instead of sticking at its last
  non-zero value (ghost backlog in Grafana/alerts). Adds a regression test.
- procrastinate: comment that _is_transient_infra_error treats all qdrant errors
  as transient deliberately (bounded same-tier retry; over-broad is acceptable).
- escalation: note next_tier is the building block; production routing uses
  ProcessorRegistry.next_available_tier.
- tests: add evaluate_escalation fast+ocr-only low-confidence -> ocr case.

Deck #323.

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

nextcloud_mcp_server/vector/queue/procrastinate.py

@@ -279,6 +279,13 @@ def _is_transient_infra_error(exc: BaseException) -> bool:
             return exc.status_code >= 500
     except ImportError:  # pragma: no cover -- openai is a hard dependency
         pass
+    # Deliberately over-broad: this treats ALL qdrant_client exceptions as
+    # transient (not just timeouts/5xx). In a healthy cluster qdrant errors are
+    # transient, and a bounded same-tier retry is cheap; a genuinely permanent
+    # qdrant fault (e.g. schema mismatch) just exhausts the transient cap and
+    # then gives up. So unlike _drop_reason (which only *labels* the cause), this
+    # may add a few retries on a non-retriable qdrant error -- an acceptable
+    # trade for not having to enumerate qdrant's non-retriable status codes.
     if type(exc).__module__.startswith("qdrant_client"):
         return True
     return False