feat(ingest): split OCR into tier2 in-cluster (GPU, gateway-only) + tier3 upstream

Insert a configurable in-cluster OCR rung into the escalation ladder (Deck #353):
a tier2-eligible doc is OCR'd on the on-demand burst GPU before falling through to
paid upstream OCR. The in-cluster backend is reached ONLY via the embedding gateway
(model prefix routes to the GPU over the tailnet) and is a config value (default
surya/surya-ocr-2, swappable to e.g. lightonocr) — never hard-coded.

Ladder: fast -> structured -> ocr-incluster -> ocr-upstream
(queues ingest-ocr-incluster / ingest-ocr-upstream).

- escalation.py: 4-tier ladder; in-cluster flag folded into the dead-letter signature.
- ocr.py: OcrProcessor(name, tier, model_setting, gateway_only); build_ocr_backend(
  ..., model=, gateway_only=) — gateway_only forces the gateway backend (never the
  direct Mistral fallback), disabling the tier with a warning if no gateway URL.
- registry.py: per-rung enable map; scanned docs target minimum="ocr-incluster";
  inline path runs the cheapest available OCR rung.
- procrastinate.py: two OCR queues; legacy ingest-ocr kept as a drain target.
- config.py: DOCUMENT_OCR_INCLUSTER_ENABLED (off) + DOCUMENT_OCR_INCLUSTER_MODEL.
- __init__.py: register the two OCR instances; vector/processor.py: pages_ocr
  metered for the upstream (paid) rung only; cli.py: new --tier choices + legacy drain.
- metrics.py: zero the legacy ingest-ocr queue gauge during rollout.
- tests: migrated to the split ladder + new tests (gateway-only forcing, per-tier
  model incl. lightonocr override, no-hard-coded-surya guard). 1792 pass; ruff + ty green.

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

tests/unit/vector/test_procrastinate_producer.py

@@ -115,7 +115,7 @@ class TestProcessDocumentTask:
         # Calling the Task runs its wrapped function in-process. The job is on the
         # ocr queue, so the queue-aware task must derive tier="ocr".
         await pq.process_document_task(
-            _ctx(pq.INGEST_QUEUE_OCR),
+            _ctx(pq.INGEST_QUEUE_OCR_UPSTREAM),
             user_id="alice",
             doc_id="42",
             doc_type="note",
@@ -131,7 +131,7 @@ class TestProcessDocumentTask:
         # Worker disables the in-process retry loop; durable retry is the queue's.
         assert captured["max_retries"] == 1
         # Tier is derived from the job's queue (escalation enabled by default).
-        assert captured["tier"] == "ocr"
+        assert captured["tier"] == "ocr-upstream"
         fake_client.close.assert_awaited_once()
 
     async def test_pipeline_error_propagates_and_closes_client(self, monkeypatch):