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>
