Previously the filler only flagged albums whose DB thumb_url was empty and, on
apply, only updated that DB thumb_url — so albums whose files had no embedded
art and no cover.jpg (but whose DB row had a URL) were never found, and even
'applying' art never touched the files. That's the reported 'doesn't scan all
albums' gap.
New core.metadata.art_apply (reuses the post-processing standard so the user's
album_art_order is honored):
- album_has_art_on_disk(): cheap-first check — folder cover.jpg/folder.jpg
sidecar, then embedded art in a representative track (FLAC/ID3/MP4/Vorbis).
- apply_art_to_album_files(): embeds via embed_album_art_metadata + writes
cover.jpg via download_cover_art; only ADDS art (never rewrites the user's
tags); read-only/unwritable files are skipped + counted, never crash.
Scan now examines every titled album and flags it when art is missing in the DB
OR on disk. Apply embeds into the album's audio files + writes cover.jpg in
addition to the DB thumbnail (media-server-only albums fall back to DB-only).
Tests cover sidecar/embedded detection, the cheap-first short-circuit, and the
apply orchestration (embeds each file + cover.jpg; read-only failures counted).
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