artworkAccessible checked every kind except radio, which fell through to
the default "no per-user access control" branch and returned true without
a lookup. Radio deletion removes only the entity row -- item_artwork and
the uploaded file survive until the next prune -- so the old ra- id kept
serving the removed radio's image for up to a day.
This is a regression against master, where newRadioArtworkReader loaded
the radio first and returned its error, so deletion took effect at once.
Radios stay globally visible; only existence is checked.
CreateMockedRadioRepo left Data nil, so its own Put panicked on first
use; initialized it.
Reported by Codex on #5847.
0669f23471