fix(ocr): round-2 review — lazy store lock, mode enum normalization, type hints

Round 2 review (PR #910):
- BLOCKING: BatchOcrJobStore._shared_lock is now lazy-init (anyio.Lock | None,
  created on first shared() call) instead of at class-definition time — matches
  the CLAUDE.md "no anyio primitives at import time" rule and OcrProcessor's
  pattern. The None-check->assign has no await between, so it's race-free.
- document_ocr_mode now normalizes via _enum_fields (case-insensitive, like
  document_ocr_provider) instead of a strict dynaconf is_in Validator, so
  DOCUMENT_OCR_MODE=Batch normalizes to "batch" rather than erroring. Tests for
  case-normalization + invalid-value rejection.
- TYPE_CHECKING-gated GatewayBatchOcrClient import so build_gateway_batch_client
  / _get_batch_client are typed `GatewayBatchOcrClient | None` instead of Any
  (runtime import stays lazy to avoid the import cycle).
- Rename ocr_options -> doc_identity_options (it's threaded to all tiers; only
  OCR reads it) + clarify the comment.
- Drop the redundant forward-ref quotes on _shared_instance.
- Add direct _batch_identity unit tests (partial/empty options branches).

Left as follow-up: reusing one httpx.AsyncClient across submit/poll (same
per-call pattern as the existing sync _GatewayOcrBackend; no clean aclose hook
on the cached client today).

1653 unit tests pass; ruff + ty green.

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

tests/unit/test_config.py

@@ -137,6 +137,19 @@ class TestGetSettings:
         assert settings.document_ocr_batch_poll_seconds == 45
         assert settings.document_ocr_batch_max_wait_seconds == 3600
 
+    @patch.dict(os.environ, {"DOCUMENT_OCR_MODE": "Batch"}, clear=True)
+    def test_document_ocr_mode_case_normalised(self):
+        """DOCUMENT_OCR_MODE is case-insensitive (normalised in __post_init__ via
+        _enum_fields, like DOCUMENT_OCR_PROVIDER) — "Batch" -> "batch"."""
+        _reload_config()
+        assert get_settings().document_ocr_mode == "batch"
+
+    @patch.dict(os.environ, {"DOCUMENT_OCR_MODE": "bogus"}, clear=True)
+    def test_document_ocr_mode_invalid_rejected(self):
+        _reload_config()
+        with pytest.raises(ValueError, match="DOCUMENT_OCR_MODE"):
+            get_settings()
+
     @patch.dict(
         os.environ,
         {"QDRANT_LOCATION": "/app/data/qdrant"},