fix(recommendation): Rediscover no longer ships a one-song playlist (#1246)

Confirmed against prod: exactly one track (17 plays, cold since May 21)
met the c>=5 + 30d-cold bar, and three process defects turned that into
a 1-track playlist instead of the locked placeholder.

- ListRediscoverTracks: collapse the two-tier UNION into one blended
  pool. The old shallow-tier gate (WHERE NOT EXISTS deep) was
  all-or-nothing — one 6-month row suppressed the entire 30-day tier —
  and deep was a strict subset of shallow anyway. Eligibility drops to
  >=3 non-skip plays (on a weeks-old history the >=5-play tracks are
  precisely the ones still in rotation); ordering prefers >=6mo cold,
  then >=5 plays, then raw count.
- Minimum viable mix floor for all five discovery mixes: below
  minLen (15; 5 for the album-coherent NewForYou/FirstListens) the
  variant is withheld so Home renders the 'listen more to unlock'
  placeholder instead of a mix that reads as built-wrong.
- /api/events: clamp client-supplied 'at' to [user.created_at,
  now+5m]. Unbounded client clocks could write arbitrarily old plays
  and poison the 6-month ordering (prod data verified clean — no
  scrub needed).

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

internal/db/dbq/system_mixes.sql.go

@@ -262,37 +262,19 @@ WITH stats AS (
       FROM play_events pe
      WHERE pe.user_id = $1 AND pe.was_skipped = false
      GROUP BY pe.track_id
-),
-deep AS (
-    SELECT t.id, t.album_id, t.artist_id, s.c, 0 AS tier
-      FROM tracks t
-      JOIN stats s ON s.track_id = t.id
-     WHERE s.c >= 5
-       AND s.last_at <= now() - interval '6 months'
-       AND NOT EXISTS (
-           SELECT 1 FROM lidarr_quarantine q
-            WHERE q.user_id = $1 AND q.track_id = t.id
-       )
-),
-shallow AS (
-    SELECT t.id, t.album_id, t.artist_id, s.c, 1 AS tier
-      FROM tracks t
-      JOIN stats s ON s.track_id = t.id
-     WHERE s.c >= 5
-       AND s.last_at <= now() - interval '30 days'
-       AND NOT EXISTS (
-           SELECT 1 FROM lidarr_quarantine q
-            WHERE q.user_id = $1 AND q.track_id = t.id
-       )
 )
-SELECT id, album_id, artist_id
-  FROM (
-    SELECT id, album_id, artist_id, c, tier FROM deep
-    UNION ALL
-    SELECT id, album_id, artist_id, c, tier FROM shallow
-     WHERE NOT EXISTS (SELECT 1 FROM deep)
-  ) u
- ORDER BY tier, c DESC, id
+SELECT t.id, t.album_id, t.artist_id
+  FROM tracks t
+  JOIN stats s ON s.track_id = t.id
+ WHERE s.c >= 3
+   AND s.last_at <= now() - interval '30 days'
+   AND NOT EXISTS (
+       SELECT 1 FROM lidarr_quarantine q
+        WHERE q.user_id = $1 AND q.track_id = t.id
+   )
+ ORDER BY (s.last_at <= now() - interval '6 months') DESC,
+          (s.c >= 5) DESC,
+          s.c DESC, t.id
  LIMIT 200
 `
 
@@ -302,13 +284,17 @@ type ListRediscoverTracksRow struct {
 	ArtistID pgtype.UUID
 }
 
-// #420 Rediscover: tracks the user played a lot (>=5 non-skip) but
-// has drifted away from. Tiered so a young library still gets a mix:
-//
-//	tier 0  not played in the last 6 months (true rediscovery)
-//	tier 1  (only if tier 0 empty) not played in the last 30 days
-//
-// Ordered by historical affection. $1 user_id.
+// #420 Rediscover: tracks the user played a lot but has drifted away
+// from. One blended pool (issue #1246: the old two-tier UNION was
+// all-or-nothing — a single ">=6 months cold" row suppressed the whole
+// ">=30 days" tier, which is how a one-song playlist shipped; the 6mo
+// tier was also a strict subset of the 30d tier). Eligibility is >=3
+// non-skip plays and >=30 days cold; the bar sits at 3 rather than 5
+// so a weeks-old library still fields a pool — on young histories the
+// >=5-play tracks are precisely the ones still in rotation. Ordering
+// prefers true rediscoveries (>=6 months cold), then strong affection
+// (>=5 plays), then raw play count; the producer's minimum floor
+// decides whether the pool is big enough to ship at all. $1 user_id.
 func (q *Queries) ListRediscoverTracks(ctx context.Context, userID pgtype.UUID) ([]ListRediscoverTracksRow, error) {
 	rows, err := q.db.Query(ctx, listRediscoverTracks, userID)
 	if err != nil {

internal/db/queries/system_mixes.sql

@@ -37,47 +37,35 @@ SELECT t.id, t.album_id, t.artist_id
  LIMIT 200;
 
 -- name: ListRediscoverTracks :many
--- #420 Rediscover: tracks the user played a lot (>=5 non-skip) but
--- has drifted away from. Tiered so a young library still gets a mix:
---   tier 0  not played in the last 6 months (true rediscovery)
---   tier 1  (only if tier 0 empty) not played in the last 30 days
--- Ordered by historical affection. $1 user_id.
+-- #420 Rediscover: tracks the user played a lot but has drifted away
+-- from. One blended pool (issue #1246: the old two-tier UNION was
+-- all-or-nothing — a single ">=6 months cold" row suppressed the whole
+-- ">=30 days" tier, which is how a one-song playlist shipped; the 6mo
+-- tier was also a strict subset of the 30d tier). Eligibility is >=3
+-- non-skip plays and >=30 days cold; the bar sits at 3 rather than 5
+-- so a weeks-old library still fields a pool — on young histories the
+-- >=5-play tracks are precisely the ones still in rotation. Ordering
+-- prefers true rediscoveries (>=6 months cold), then strong affection
+-- (>=5 plays), then raw play count; the producer's minimum floor
+-- decides whether the pool is big enough to ship at all. $1 user_id.
 WITH stats AS (
     SELECT pe.track_id, COUNT(*) AS c, MAX(pe.started_at) AS last_at
       FROM play_events pe
      WHERE pe.user_id = $1 AND pe.was_skipped = false
      GROUP BY pe.track_id
-),
-deep AS (
-    SELECT t.id, t.album_id, t.artist_id, s.c, 0 AS tier
-      FROM tracks t
-      JOIN stats s ON s.track_id = t.id
-     WHERE s.c >= 5
-       AND s.last_at <= now() - interval '6 months'
-       AND NOT EXISTS (
-           SELECT 1 FROM lidarr_quarantine q
-            WHERE q.user_id = $1 AND q.track_id = t.id
-       )
-),
-shallow AS (
-    SELECT t.id, t.album_id, t.artist_id, s.c, 1 AS tier
-      FROM tracks t
-      JOIN stats s ON s.track_id = t.id
-     WHERE s.c >= 5
-       AND s.last_at <= now() - interval '30 days'
-       AND NOT EXISTS (
-           SELECT 1 FROM lidarr_quarantine q
-            WHERE q.user_id = $1 AND q.track_id = t.id
-       )
 )
-SELECT id, album_id, artist_id
-  FROM (
-    SELECT id, album_id, artist_id, c, tier FROM deep
-    UNION ALL
-    SELECT id, album_id, artist_id, c, tier FROM shallow
-     WHERE NOT EXISTS (SELECT 1 FROM deep)
-  ) u
- ORDER BY tier, c DESC, id
+SELECT t.id, t.album_id, t.artist_id
+  FROM tracks t
+  JOIN stats s ON s.track_id = t.id
+ WHERE s.c >= 3
+   AND s.last_at <= now() - interval '30 days'
+   AND NOT EXISTS (
+       SELECT 1 FROM lidarr_quarantine q
+        WHERE q.user_id = $1 AND q.track_id = t.id
+   )
+ ORDER BY (s.last_at <= now() - interval '6 months') DESC,
+          (s.c >= 5) DESC,
+          s.c DESC, t.id
  LIMIT 200;
 
 -- name: ListNewForYouTracks :many