Address PR #863 round 4: classify_from_text's docstring now states that the
image_heavy flag (and the image-coverage trigger) are only set when
image_coverage is supplied, so the flag reads zero for tenants with
DOCUMENT_OCR_DETECT_SCANNED=false -- self-documenting the metric semantics.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>