fix(ingest): warn when provider=none disables in-cluster; precise model fallback

Address claude-review round 4 on #922:

- Important 1: build_ocr_backend now warns when gateway_only + provider=none +
  DOCUMENT_OCR_INCLUSTER_ENABLED=true — provider=none suppresses the gateway-only
  in-cluster tier too (it never uses the mistral provider), which surprises an
  operator who set none just to disable Mistral. Restructured so the gateway_only
  branch is evaluated before the generic provider=none return. Two tests cover
  the warn-when-enabled / silent-when-disabled cases.
- Nit 3: model fallback uses `model if model is not None else ...` (not `or`), so
  an empty model string no longer silently falls back to the upstream default and
  misroutes a per-tier rung. Test added.
- Nit 4: the no-surya-literal guard now uses rglob so future
  document_processors/ subdirs are covered.

Deferred (pre-existing / out of scope for this PR):
- Important 2 (_GatewayOcrBackend opens a new httpx.AsyncClient per ocr() call):
  a pre-existing pattern affecting both OCR rungs; a shared pooled client needs
  careful per-pod lifecycle handling (event-loop binding, aclose) and is better
  as its own change. Follow-up.
- Nit 5 (_MANAGED_QUEUES vs the CLI all-queues list): the two sets differ
  deliberately (the CLI list includes ingest-maintenance, _MANAGED_QUEUES does
  not), so a shared constant wouldn't cleanly dedupe them.

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

nextcloud_mcp_server/document_processors/ocr.py

@@ -239,11 +239,22 @@ def build_ocr_backend(
     disabled (warn) rather than misrouted to a direct backend that can't serve it.
     """
     provider = settings.document_ocr_provider
-    model = model or settings.document_ocr_model
-    if provider == "none":
-        return None
+    # `is not None` (not truthiness): an empty model string must NOT silently fall
+    # back to the upstream default and misroute a per-tier rung.
+    model = model if model is not None else settings.document_ocr_model
 
     if gateway_only:
+        # The in-cluster tier is gateway-only and never touches the `mistral`
+        # provider, but provider=none still suppresses it. Warn so an operator who
+        # set provider=none to disable Mistral doesn't silently lose the GPU tier.
+        if provider == "none":
+            if settings.document_ocr_incluster_enabled:
+                logger.warning(
+                    "DOCUMENT_OCR_PROVIDER=none disables in-cluster OCR even with "
+                    "DOCUMENT_OCR_INCLUSTER_ENABLED=true; set it to 'gateway' or "
+                    "'auto' to keep the in-cluster tier"
+                )
+            return None
         if settings.embedding_gateway_url:
             return _GatewayOcrBackend(
                 settings.embedding_gateway_url,
@@ -256,6 +267,9 @@ def build_ocr_backend(
         )
         return None
 
+    if provider == "none":
+        return None
+
     if provider in ("gateway", "auto") and settings.embedding_gateway_url:
         return _GatewayOcrBackend(
             settings.embedding_gateway_url,