fix(review): lock OCR backend init, warn on rollback fallthrough, zero-page metric

Address PR #858 review round 2:

- OcrProcessor backend resolution is now guarded by an anyio.Lock (lazy-init,
  double-checked) so a burst of concurrent first-OCR calls resolves the backend
  once instead of each fetching its own gateway M2M token.
- The document_tier1_engine=pymupdf rollback now logs a warning when it falls
  back to the fast processor (no 'structured' registered) instead of silently
  using the very engine the operator opted out of.
- classify_from_text defaults ocr_frac to 0.0 (not 1.0) for a zero-page PDF, so
  the recorded classification metric is "fast" (no OCR evidence) rather than a
  misleading "ocr"; the no_text_layer/bad_text_layer flags are gated on having
  sampled at least one page.

New tests: zero-page classify routes fast, rollback-fallback warning.

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

nextcloud_mcp_server/document_processors/ocr.py

@@ -22,6 +22,7 @@ from abc import ABC, abstractmethod
 from collections.abc import Awaitable, Callable
 from typing import Any
 
+import anyio
 import httpx
 
 from nextcloud_mcp_server.config import Settings, get_settings
@@ -184,6 +185,10 @@ class OcrProcessor(DocumentProcessor):
         # lifetime is safe.
         self._backend_resolved = False
         self._backend: _OcrBackend | None = None
+        # Serialise first-call resolution so a burst of concurrent OCR requests
+        # doesn't each build a backend (and fetch its own M2M token). Lazy-init:
+        # anyio primitives must not be created at import time.
+        self._backend_lock: anyio.Lock | None = None
 
     @property
     def name(self) -> str:
@@ -209,8 +214,12 @@ class OcrProcessor(DocumentProcessor):
     ) -> ProcessingResult:
         settings = get_settings()
         if not self._backend_resolved:
-            self._backend = build_ocr_backend(settings)
-            self._backend_resolved = True
+            if self._backend_lock is None:
+                self._backend_lock = anyio.Lock()
+            async with self._backend_lock:
+                if not self._backend_resolved:  # double-checked
+                    self._backend = build_ocr_backend(settings)
+                    self._backend_resolved = True
         backend = self._backend
         if backend is None:
             logger.warning(