fix(playlists): Songs-like mixes no longer vanish after a quiet week

PickSeedArtists had a hard 7-day window with no fallback: a week
without listening emptied the seed pool, produceSeedMixes returned
zero playlists, and the daily atomic-replace build deleted every
existing "Songs like X" mix until the user played something again
(#1255).

The query now falls back through widening engagement windows — 7d →
30d → all-time → liked artists — the same tiered shape that fixed the
identical vanish for For You's seeds (PickTopPlayedTracksForUser).
Like-boost scoring is preserved in every tier.

All returned rows share the winning tier, and produceSeedMixes maps it
onto the rule-#131 pick-kind ladder (7d = tier1 exact, 30d = tier2,
all-time/liked = tier3) and stamps the built tracks — the #1270
provenance pipeline then attributes plays and skips to seed freshness,
so the metrics card can say whether stale-seeded mixes actually
perform worse.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TsF3cNoKrqCYsU78cXC8U6

internal/playlists/system.go

@@ -147,8 +147,28 @@ const (
 	pickKindDormant   = "dormant"
 	pickKindCrossUser = "cross_user"
 	pickKindRandom    = "random"
+	pickKindTier1     = "tier1"
+	pickKindTier2     = "tier2"
+	pickKindTier3     = "tier3"
 )
 
+// pickKindForSeedTier maps PickSeedArtists' seed-pool fallback tier
+// onto the rule-#131 pick-kind ladder: fresh 7-day engagement pins the
+// mix's exact desire (tier1), the 30-day window steps back a little
+// (tier2), and all-time / liked-only seeds are the far fallback
+// (tier3). Stamped mix-wide — seed staleness is a property of the
+// whole build, and it's what the metrics breakdown attributes skips to.
+func pickKindForSeedTier(tier int32) string {
+	switch tier {
+	case 0:
+		return pickKindTier1
+	case 1:
+		return pickKindTier2
+	default:
+		return pickKindTier3
+	}
+}
+
 const systemMixLength = 25
 
 // systemMixWeights are the fixed scoring weights used by the cron worker.
@@ -389,8 +409,11 @@ func produceForYou(
 }
 
 // produceSeedMixes: up to 3 "Songs like {artist}" mixes. Seed
-// artists rotate daily-deterministically. The base seed-artist
-// query failing is fatal; per-artist failures are logged + skipped.
+// artists rotate daily-deterministically; the seed query falls back
+// through widening engagement windows (#1255) and every returned row
+// shares the winning tier, stamped onto the built tracks as their
+// pick_kind. The base seed-artist query failing is fatal; per-artist
+// failures are logged + skipped.
 func produceSeedMixes(
 	ctx context.Context, q *dbq.Queries, logger *slog.Logger,
 	userID pgtype.UUID, dateStr string, now time.Time,
@@ -399,6 +422,10 @@ func produceSeedMixes(
 	if err != nil {
 		return nil, fmt.Errorf("pick seed artists: %w", err)
 	}
+	seedTierKind := ""
+	if len(seedRows) > 0 {
+		seedTierKind = pickKindForSeedTier(seedRows[0].Tier)
+	}
 	seedRowsLocal := make([]seedArtistRow, 0, len(seedRows))
 	for _, r := range seedRows {
 		seedRowsLocal = append(seedRowsLocal, seedArtistRow{
@@ -443,6 +470,9 @@ func produceSeedMixes(
 		if len(tracks) == 0 {
 			continue
 		}
+		for i := range tracks {
+			tracks[i].PickKind = seedTierKind
+		}
 		out = append(out, builtPlaylist{
 			Name:         fmt.Sprintf("Songs like %s", artistRow.Name),
 			Variant:      "songs_like_artist",