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>

tests/unit/test_config.py

@@ -397,6 +397,14 @@ class TestDynaconfValidators:
         with pytest.raises(ValidationError, match="DOCUMENT_OCR_PAGE_FRACTION"):
             _reload_config()
 
+    @patch.dict(os.environ, {"DOCUMENT_OCR_MIN_PAGE_CHARS": "-1"}, clear=True)
+    def test_ocr_min_page_chars_negative(self):
+        """DOCUMENT_OCR_MIN_PAGE_CHARS must be non-negative."""
+        from dynaconf import ValidationError
+
+        with pytest.raises(ValidationError, match="DOCUMENT_OCR_MIN_PAGE_CHARS"):
+            _reload_config()
+
     @patch.dict(os.environ, {"LOG_LEVEL": "VERBOSE"}, clear=True)
     def test_invalid_log_level(self):
         """Test invalid LOG_LEVEL raises ValidationError."""