fix(review): align quality threshold, cap + DRY scan coverage, warn on failure

Address PR #863 review:

- MIN_TEXT_QUALITY 0.45 -> 0.5 so the module/diagnostic default matches the
  DOCUMENT_OCR_MIN_TEXT_QUALITY setting (registry always passes the setting; this
  keeps classify_pdf and the test/default path on the production threshold).
- image_coverage_per_page is bounded to MAX_SAMPLED_PAGES (the image pass is the
  costly part, so a 200-page scan isn't fully rasterised on the hot path); pages
  beyond the cap fall back to the text-quality signal, and page_fraction still
  gates over every page.
- Extracted _page_image_coverage(page) helper, shared by classify_pdf and
  image_coverage_per_page (DRY + keeps the tiling-double-count note in one place).
- Scan-detection failure logs at WARNING (not DEBUG) so a systematic failure on
  an OCR-enabled tenant is visible at LOG_LEVEL=INFO.
- Add the missing DOCUMENT_OCR_MIN_PAGE_CHARS range-validator test.

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

nextcloud_mcp_server/document_processors/registry.py

@@ -248,7 +248,10 @@ class ProcessorRegistry:
                     try:
                         image_coverage = image_coverage_per_page(content)
                     except Exception:
-                        logger.debug(
+                        # Best-effort: fall back to text-only signals. WARNING
+                        # (not DEBUG) so a systematic scan-detection failure on an
+                        # OCR-enabled tenant is visible at LOG_LEVEL=INFO.
+                        logger.warning(
                             "Scan detection failed for %s; using text-only signals",
                             filename or "<bytes>",
                             exc_info=True,