feat(server/playlists): Discover system playlist

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>

internal/playlists/system.go

@@ -226,6 +226,14 @@ func BuildSystemPlaylists(ctx context.Context, pool *pgxpool.Pool, logger *slog.
 		})
 	}
 
+	// 4. Discover: surface unheard tracks, biased toward dormant artists.
+	discoverTracks, derr := buildDiscoverCandidates(ctx, q, userID, dateStr)
+	if derr != nil {
+		logger.Warn("system playlist: discover candidates load failed; skipping",
+			"user_id", uuidStringPL(userID), "err", derr)
+		discoverTracks = nil
+	}
+
 	// 3. Atomic replace inside a transaction.
 	tx, err := pool.Begin(ctx)
 	if err != nil {
@@ -270,6 +278,16 @@ func BuildSystemPlaylists(ctx context.Context, pool *pgxpool.Pool, logger *slog.
 		createdIDs = append(createdIDs, id)
 	}
 
+	if len(discoverTracks) > 0 {
+		id, err := insertSystemPlaylist(ctx, qtx, userID, "Discover", "discover",
+			pgtype.UUID{}, discoverTracks)
+		if err != nil {
+			buildErr = fmt.Errorf("insert discover: %w", err)
+			return buildErr
+		}
+		createdIDs = append(createdIDs, id)
+	}
+
 	if err := tx.Commit(ctx); err != nil {
 		buildErr = fmt.Errorf("commit tx: %w", err)
 		return buildErr