.docx is accepted for ingest but the format-group switch had no case
for it, so every DOCX row stored format_group 'unknown' with
is_reflowable=false. Extract the classification switch into a pure
classifyFormatGroup helper with a table test, and classify .docx
reflowable like the other reflowable text formats (.rtf/.doc stay out
until they are readable client-side). Existing rows: dev alice (DOCX)
reclassified by hand; production rows reclassify on rescan.