Follow-up to the audio-only feed guard. Real case: a Nodesignal-Talk podcast
published with `imeta m audio/mp4` + a `.f4a` URL. It was already filtered via
its imeta hint (audio/mp4 → hasAudio), but unlike the .ogg/.mp3 ones it actually
PLAYS in a <video> element (AAC-in-MP4 = audio + poster), which is why it looked
like it "worked." It's still a podcast, so it stays out of the video grid.
Closes the fallback gap for audio/mp4 notes that lack an imeta audio hint: added
.f4a/.m4b/.mka to AUDIO_ONLY_URL_PATTERN (AAC-in-MP4 audio / audiobooks /
Matroska audio). .mp4/.m4v remain video and are explicitly asserted not to match.
Tests: video-schema-and-conversion (+2, 13/13). Lint + build clean. TODO #60.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
