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/db/queries/system_playlists.sql

@@ -47,14 +47,29 @@ UPDATE system_playlist_runs
 UPDATE system_playlist_runs SET in_flight = false WHERE in_flight = true;
 
 -- name: PickSeedArtists :many
--- Top-5 most-engaged distinct artist candidates in the user's last 7
--- days. The Go-side picker (pickSeedArtistsForDay) shuffles these
--- daily-deterministically and takes the first 3 so the set of
--- "Songs like X" mixes rotates day-to-day.
--- Score = unskipped-play count + 5 if user has liked the artist.
-WITH plays AS (
+-- Top-5 most-engaged distinct artist candidates, tiered so the
+-- "Songs like X" mixes never silently vanish (#1255): the old hard
+-- 7-day window emptied the seed pool after a quiet week, and the
+-- daily atomic-replace build then deleted every existing mix until
+-- the user played something again. Same fallback shape as
+-- PickTopPlayedTracksForUser (For You's seeds):
+--   tier 0  engagement in the last 7 days
+--   tier 1  (only if tier 0 empty) last 30 days
+--   tier 2  (only if tiers 0+1 empty) all-time
+--   tier 3  (only if 0-2 empty) liked artists with no play history
+-- All returned rows share one tier; produceSeedMixes maps it onto the
+-- rule-#131 pick-kind ladder and stamps the built tracks, so metrics
+-- can compare mixes seeded from fresh vs stale engagement.
+-- Score = unskipped-play count + 5 if user has liked the artist. The
+-- Go-side picker (pickSeedArtistsForDay) shuffles the 5
+-- daily-deterministically and takes 3 so the mix set rotates.
+WITH liked AS (
+    SELECT gla.artist_id FROM general_likes_artists gla WHERE gla.user_id = $1
+),
+recent7 AS (
     SELECT t.artist_id,
-           COUNT(*) FILTER (WHERE pe.was_skipped = false) AS play_count
+           COUNT(*) FILTER (WHERE pe.was_skipped = false) AS play_count,
+           0 AS tier
       FROM play_events pe
       JOIN tracks t ON t.id = pe.track_id
      WHERE pe.user_id = $1
@@ -62,14 +77,52 @@ WITH plays AS (
        AND t.artist_id IS NOT NULL
      GROUP BY t.artist_id
 ),
-liked AS (
-    SELECT artist_id FROM general_likes_artists WHERE user_id = $1
+recent30 AS (
+    SELECT t.artist_id,
+           COUNT(*) FILTER (WHERE pe.was_skipped = false) AS play_count,
+           1 AS tier
+      FROM play_events pe
+      JOIN tracks t ON t.id = pe.track_id
+     WHERE pe.user_id = $1
+       AND pe.started_at > now() - INTERVAL '30 days'
+       AND t.artist_id IS NOT NULL
+     GROUP BY t.artist_id
+),
+alltime AS (
+    SELECT t.artist_id,
+           COUNT(*) FILTER (WHERE pe.was_skipped = false) AS play_count,
+           2 AS tier
+      FROM play_events pe
+      JOIN tracks t ON t.id = pe.track_id
+     WHERE pe.user_id = $1
+       AND t.artist_id IS NOT NULL
+     GROUP BY t.artist_id
+),
+likedonly AS (
+    SELECT l.artist_id, 0::bigint AS play_count, 3 AS tier
+      FROM liked l
+),
+chosen AS (
+    SELECT artist_id, play_count, tier FROM recent7
+    UNION ALL
+    SELECT artist_id, play_count, tier FROM recent30
+     WHERE NOT EXISTS (SELECT 1 FROM recent7)
+    UNION ALL
+    SELECT artist_id, play_count, tier FROM alltime
+     WHERE NOT EXISTS (SELECT 1 FROM recent7)
+       AND NOT EXISTS (SELECT 1 FROM recent30)
+    UNION ALL
+    SELECT artist_id, play_count, tier FROM likedonly
+     WHERE NOT EXISTS (SELECT 1 FROM recent7)
+       AND NOT EXISTS (SELECT 1 FROM recent30)
+       AND NOT EXISTS (SELECT 1 FROM alltime)
 )
-SELECT p.artist_id,
-       (p.play_count + CASE WHEN l.artist_id IS NOT NULL THEN 5 ELSE 0 END)::bigint AS score
-  FROM plays p
-  LEFT JOIN liked l ON l.artist_id = p.artist_id
- ORDER BY score DESC, p.artist_id
+SELECT c.artist_id,
+       (c.play_count + CASE WHEN l.artist_id IS NOT NULL THEN 5 ELSE 0 END)::bigint AS score,
+       c.tier::int AS tier
+  FROM chosen c
+  LEFT JOIN liked l ON l.artist_id = c.artist_id
+ ORDER BY score DESC, c.artist_id
  LIMIT 5;
 
 -- name: PickTopPlayedTracksForUser :many