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/document_processors/escalation.py

@@ -30,10 +30,11 @@ def next_tier(current: str) -> str | None:
     """The next tier above ``current`` in the ladder, or ``None`` if terminal.
 
     Pure ordering only -- it does not consider whether the next tier is
-    *available* (a processor registered / OCR enabled). Callers that need
-    availability resolve it against the registry + settings (see
-    ``ProcessorRegistry.next_available_tier``); a tier with no escalation target
-    is terminal and its result is indexed as-is.
+    *available* (a processor registered / OCR enabled). **Production routing uses
+    ``ProcessorRegistry.next_available_tier``**, which layers availability on top
+    of this ordering; ``next_tier`` itself is the underlying building block
+    (referenced directly by tests). A tier with no escalation target is terminal
+    and its result is indexed as-is.
     """
     try:
         idx = TIER_LADDER.index(current)