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/seed_selection_test.go

@@ -46,6 +46,26 @@ func TestPickSeedArtistsFromRows_Empty(t *testing.T) {
 	}
 }
 
+func TestPickKindForSeedTier(t *testing.T) {
+	// The seed query's fallback tiers map onto the rule-#131 ladder:
+	// fresh 7-day engagement is the exact desire, everything past the
+	// 30-day step-back collapses into the far tier (#1255).
+	cases := []struct {
+		tier int32
+		want string
+	}{
+		{0, pickKindTier1},
+		{1, pickKindTier2},
+		{2, pickKindTier3},
+		{3, pickKindTier3},
+	}
+	for _, c := range cases {
+		if got := pickKindForSeedTier(c.tier); got != c.want {
+			t.Errorf("pickKindForSeedTier(%d) = %q, want %q", c.tier, got, c.want)
+		}
+	}
+}
+
 // userIDHash is the per-user, per-day hash that drives the daily-
 // determinism RNGs. Same family as tieBreakHash, just keyed on user
 // ID instead of track ID.