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>

tests/unit/test_registry_tiering.py

@@ -400,3 +400,23 @@ def test_evaluate_escalation_zero_page_does_not_escalate(monkeypatch):
         processor="fast",
     )
     assert r.evaluate_escalation(res, b"%PDF", "fast", _Settings(ocr=True)) is None
+
+
+def test_evaluate_escalation_lowconf_to_ocr_when_no_structured(monkeypatch):
+    """fast+ocr only: a low-confidence parse routes straight to ocr (skips the
+    unregistered structured rung), not to None."""
+    monkeypatch.setattr(reg_mod, "record_document_classification", MagicMock())
+    junk = "z" * 40  # non-empty but junk -> recommended ocr, total_chars > 0
+    r = _registry((_Fake("fast", "fast"), 20), (_Fake("ocr", "ocr"), 5))
+    res = ProcessingResult(
+        text=junk,
+        metadata={
+            "page_count": 1,
+            "page_boundaries": [
+                {"page": 1, "start_offset": 0, "end_offset": len(junk)}
+            ],
+        },
+        processor="fast",
+    )
+    decision = r.evaluate_escalation(res, b"%PDF", "fast", _Settings(ocr=True))
+    assert decision == ("ocr", "low_confidence")