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>

nextcloud_mcp_server/config.py

@@ -366,7 +366,10 @@ _dynaconf = Dynaconf(
         Validator("DOCUMENT_CHUNK_SIZE", gte=1),
         Validator("DOCUMENT_PARSE_TIMEOUT_SECONDS", gte=1),
         Validator("DOCUMENT_OCR_TIMEOUT_SECONDS", gte=1),
-        Validator("DOCUMENT_OCR_MODE", is_in=("sync", "batch")),
+        # DOCUMENT_OCR_MODE is normalised + membership-checked in
+        # Settings.__post_init__ via _enum_fields (case-insensitive, like
+        # DOCUMENT_OCR_PROVIDER) — no strict dynaconf Validator here, so
+        # "Batch"/"SYNC" normalise instead of erroring.
         # Poll cadence well above a few seconds (each poll re-runs the tier);
         # deadline at least one poll interval.
         Validator("DOCUMENT_OCR_BATCH_POLL_SECONDS", gte=5),
@@ -1029,6 +1032,7 @@ class Settings:
             "collection_metadata_source": {"qdrant", "api"},
             "document_tier1_engine": {"pypdfium2", "pymupdf"},
             "document_ocr_provider": {"auto", "gateway", "mistral", "none"},
+            "document_ocr_mode": {"sync", "batch"},
         }
         for _field, _allowed in _enum_fields.items():
             _val = (getattr(self, _field) or "").strip().lower()