refactor(ingest): doc legacy ocr queue; fix docstrings + getattr guard

Address claude-review round 2 on #922:

- Legacy `ingest-ocr` tier resolution (important 1): document why it deliberately
  resolves to `fast` rather than mapping to `ocr-upstream` — stranded pre-split
  jobs re-extract empty and re-escalate via the ladder to the cheap
  `ocr-incluster` rung, keeping them OFF the paid upstream rung. Added a
  tier_for_queue(LEGACY_INGEST_QUEUE_OCR) == "fast" assertion.
- Double get_settings() in `_get_batch_client` (important 2): bind once to a local.
- Stale docstrings (important 3): OcrProcessor (serves both rungs now),
  _tier_available (both OCR rungs gated), evaluate_escalation (targets
  ocr-incluster, falls through to ocr-upstream).
- Misconfigured model_setting (nit 5): OcrProcessor.__init__ raises ValueError on
  an unknown settings attr (fail-fast at startup vs AttributeError mid-OCR); also
  removes the dynamic-getattr static-analysis smell SonarCloud flagged.
- Redundant guard (nit 4): kept `and ocr_tier is not None` — it's required for ty
  to narrow ocr_tier to str for record_document_escalation; added a comment.
- Test gap (nit 6): added a test pinning the CURRENT incluster-failure ->
  tier-1 fallback (does NOT cascade to upstream) so the future 503-escalation
  change is an explicit diff.

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

tests/unit/vector/test_tiered_escalation_strategy.py

@@ -51,6 +51,10 @@ class TestLadder:
         # Legacy / unknown / None all fall back to the cheapest tier.
         assert pq.tier_for_queue(pq.LEGACY_INGEST_QUEUE) == "fast"
         assert pq.tier_for_queue(None) == "fast"
+        # The pre-split legacy OCR queue also resolves to fast (NOT ocr-upstream):
+        # stranded in-flight jobs re-extract empty and re-escalate via the ladder
+        # to the cheap ocr-incluster rung, never straight to paid upstream.
+        assert pq.tier_for_queue(pq.LEGACY_INGEST_QUEUE_OCR) == "fast"
 
 
 class TestTieredEscalationStrategy: