The two Plex arms of media-census.py — `plex` (the movie planner's title
authority) and `audiobooks` (the chapter rows the audiobook plan files by) —
opened the library `mode=ro&immutable=1` while the Jellyfin arms already used
plain `mode=ro`. Same defect as media-verify.py: an uncheckpointed row of a
running scan is invisible, so a just-scanned item is simply absent from the
authority file a plan is built from, and the pass cannot say why.
Both now go through open_plex_ro(), which tries plain ro first and keeps
immutable only as the fallback for a database whose -shm cannot be read. The
new audiobooks test leaves a chapter row committed-but-uncheckpointed and
asserts the export still carries it; red before the change, green after.