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_test.go

@@ -135,6 +135,59 @@ func TestBuildSystemPlaylists_SufficientActivity(t *testing.T) {
 	}
 }
 
+func TestBuildSystemPlaylists_StaleActivity_SongsLikeSurvives(t *testing.T) {
+	// #1255: the seed-artist query's old hard 7-day window emptied the
+	// pool after a quiet week, and the atomic-replace build then deleted
+	// every "Songs like X" mix. With the tiered fallback, plays that are
+	// ~20 days old (outside 7d, inside 30d) must still seed the mixes —
+	// and the built tracks carry the tier2 stamp so metrics can compare
+	// stale-seeded mixes against fresh ones.
+	pool := newPool(t)
+	logger := discardLogger()
+	u := seedUser(t, pool, "stale1")
+	old := time.Now().UTC().Add(-20 * 24 * time.Hour)
+	for a := 0; a < 4; a++ {
+		for k := 0; k < 3; k++ {
+			tk := seedTrack(t, pool,
+				"stale1-track-"+string(rune('A'+a))+string(rune('0'+k)),
+				"stale1-artist-"+string(rune('A'+a)))
+			for p := 0; p < 3; p++ {
+				seedPlayEvent(t, pool, u.ID, tk.ID,
+					old.Add(-time.Duration(a*10+k+p)*time.Hour), false)
+			}
+		}
+	}
+
+	if err := playlists.BuildSystemPlaylists(context.Background(), pool, logger, u.ID, time.Now().UTC(), t.TempDir()); err != nil {
+		t.Fatalf("build: %v", err)
+	}
+
+	rows, err := pool.Query(context.Background(), `
+		SELECT DISTINCT COALESCE(pt.pick_kind, '<null>')
+		  FROM playlist_tracks pt
+		  JOIN playlists p ON p.id = pt.playlist_id
+		 WHERE p.user_id = $1 AND p.system_variant = 'songs_like_artist'
+	`, u.ID)
+	if err != nil {
+		t.Fatalf("query pick_kinds: %v", err)
+	}
+	defer rows.Close()
+	var kinds []string
+	for rows.Next() {
+		var k string
+		if err := rows.Scan(&k); err != nil {
+			t.Fatalf("scan: %v", err)
+		}
+		kinds = append(kinds, k)
+	}
+	if len(kinds) == 0 {
+		t.Fatal("expected songs_like_artist tracks from the 30-day fallback tier; got none (the vanish bug)")
+	}
+	if len(kinds) != 1 || kinds[0] != "tier2" {
+		t.Errorf("pick_kinds = %v, want exactly [tier2] (30-day fallback seeds)", kinds)
+	}
+}
+
 func TestBuildSystemPlaylists_QuarantineExcluded(t *testing.T) {
 	pool := newPool(t)
 	logger := discardLogger()