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/test_processor_metering.py

@@ -191,7 +191,7 @@ async def test_ocr_tier_records_pages_ocr(store_spy):
         token_count=900,
         total_chars=40000,
         page_count=8,
-        pipeline_tier="ocr",
+        pipeline_tier="ocr-upstream",
     )
     by_metric = {
         c.kwargs["metric"]: c.kwargs["value"]
@@ -205,7 +205,7 @@ async def test_ocr_tier_records_pages_ocr(store_spy):
     }
     # pipeline_tier is threaded into the billing metadata for CP attribution.
     for c in store_spy.record_usage_event.await_args_list:
-        assert c.kwargs["metadata"]["pipeline_tier"] == "ocr"
+        assert c.kwargs["metadata"]["pipeline_tier"] == "ocr-upstream"
 
 
 @pytest.mark.unit