Reported by CubeComming: importing media keeps the track artist correct
(e.g. Billie Eilish) but changes the album-artist tag ("Albuminterpret") to
"Unknown Artist", breaking grouping in the media server.
Cause: in extract_source_metadata (core/metadata/source.py), album_artist is
seeded from the resolved track artist, then overridden by the album CONTEXT's
first artist. When the album lookup comes back unresolved, that first artist is
the literal "Unknown Artist" placeholder — which is truthy, so it clobbered the
real artist.
Fix: treat "Unknown Artist" (and empty) as a non-value — only let the album
context override the album_artist when it names a real artist. A genuine album
artist (e.g. "Various Artists") still overrides as before.
Tests: tests/metadata/test_album_artist_unknown.py — placeholder doesn't
clobber, real album artist still used, no-album-context falls back to track
artist, empty doesn't clobber. (Pre-existing test_album_mbid_cache.py failures
are an unrelated sandbox DB disk-I/O issue.)
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