Adds the third system playlist variant: 'discover'. Surfaces 100
tracks the operator has not played and not liked, biased toward
artists they rarely play (< 10 plays) and complemented by tracks
liked by other users plus a random unheard sample. Cold start
collapses to all-random; single-user servers redistribute the
cross-user-likes allocation equally across the other two buckets.
The build runs as a fourth phase inside BuildSystemPlaylists
alongside For You and the seed-artist mixes. Same daily-deterministic
md5(track_id || dateStr) ordering as the other variants. Per-album
(<=2) and per-artist (<=3) caps prevent collapse onto a single
prolific artist. Buckets interleave round-robin so the playlist
order doesn't front-load one bucket's flavour.
Post-commit collage generation (already wired) gives Discover the
same 4-cell cover treatment as the other system playlists — no
new collage code needed.
Tests cover the slot-redistribution table (all-available,
cold-start, single-user, partial-dormant, fully-empty), the
per-album/per-artist caps, the round-robin interleave, and a
DB-backed cold-start integration test that asserts a Discover
playlist lands with non-zero tracks.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
System-generated playlists ("For You", "Songs like X") rendered with
empty placeholder boxes in the UI even when their contributing tracks
had album art. The build path inserted playlist rows directly without
ever invoking the existing playlists.GenerateCollage machinery —
that was only wired into user-edited playlist mutations.
After this change, BuildSystemPlaylists generates a 4-cell collage
for every system playlist it creates, post-commit. Same cover
mechanism user playlists use, same on-disk layout
(<DataDir>/playlist_covers/<id>.jpg). Cells whose source album lacks
art fall back to the FabledSword glyph, so a partially-covered
library still produces a sensible visual.
Threading: BuildSystemPlaylists, StartSystemPlaylistCron, and the
api lazy-build path all gain a dataDir string parameter; main.go
passes cfg.Storage.DataDir.
Drops the now-redundant PickTopAlbumCoverForArtistByUser lookup —
the collage is more visually informative than a single album cover
copy, and the seed-artist's top album is one of the contributing
tracks in the mix anyway, so it'll appear as one of the four cells.
Tests pass t.TempDir() for the dataDir parameter.
bvandeusen