refactor(playlists): SQL queries return top-5 candidate seeds

For-You + Songs-Like-X seed selection moves to Go-side daily rotation
(next commit). The SQL change just widens the candidate pool: top-5
played tracks instead of single top played track; top-5 artists
instead of top-3.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/db/dbq/system_playlists.sql.go

@@ -308,7 +308,7 @@ SELECT p.artist_id,
   FROM plays p
   LEFT JOIN liked l ON l.artist_id = p.artist_id
  ORDER BY score DESC, p.artist_id
- LIMIT 3
+ LIMIT 5
 `
 
 type PickSeedArtistsRow struct {
@@ -316,7 +316,10 @@ type PickSeedArtistsRow struct {
 	Score    int64
 }
 
-// Top-3 most-engaged distinct artists in the user's last 7 days.
+// 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.
 func (q *Queries) PickSeedArtists(ctx context.Context, userID pgtype.UUID) ([]PickSeedArtistsRow, error) {
 	rows, err := q.db.Query(ctx, pickSeedArtists, userID)
@@ -408,7 +411,7 @@ func (q *Queries) PickTopPlayedTrackForArtistByUser(ctx context.Context, arg Pic
 	return id, err
 }
 
-const pickTopPlayedTrackForUser = `-- name: PickTopPlayedTrackForUser :one
+const pickTopPlayedTracksForUser = `-- name: PickTopPlayedTracksForUser :many
 SELECT t.id
   FROM play_events pe
   JOIN tracks t ON t.id = pe.track_id
@@ -417,16 +420,33 @@ SELECT t.id
    AND pe.was_skipped = false
  GROUP BY t.id
  ORDER BY COUNT(*) DESC, t.id
- LIMIT 1
+ LIMIT 5
 `
 
-// For-You seed selection. Returns the user's most-played non-skipped
-// track in the last 7 days; tie-break by track_id for determinism.
-func (q *Queries) PickTopPlayedTrackForUser(ctx context.Context, userID pgtype.UUID) (pgtype.UUID, error) {
-	row := q.db.QueryRow(ctx, pickTopPlayedTrackForUser, userID)
-	var id pgtype.UUID
-	err := row.Scan(&id)
-	return id, err
+// For-You candidate seeds. Returns the user's top-5 most-played
+// non-skipped tracks in the last 7 days; tie-break by track_id for
+// determinism. The Go-side picker (pickForYouSeedForDay) chooses one
+// of the returned rows as today's seed via userIDHash so the
+// candidate pool rotates day-to-day while staying stable within a
+// day.
+func (q *Queries) PickTopPlayedTracksForUser(ctx context.Context, userID pgtype.UUID) ([]pgtype.UUID, error) {
+	rows, err := q.db.Query(ctx, pickTopPlayedTracksForUser, userID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []pgtype.UUID
+	for rows.Next() {
+		var id pgtype.UUID
+		if err := rows.Scan(&id); err != nil {
+			return nil, err
+		}
+		items = append(items, id)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
 }
 
 const tryClaimSystemPlaylistRun = `-- name: TryClaimSystemPlaylistRun :one

internal/db/queries/system_playlists.sql

@@ -47,7 +47,10 @@ UPDATE system_playlist_runs
 UPDATE system_playlist_runs SET in_flight = false WHERE in_flight = true;
 
 -- name: PickSeedArtists :many
--- Top-3 most-engaged distinct artists in the user's last 7 days.
+-- 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 (
     SELECT t.artist_id,
@@ -67,11 +70,15 @@ SELECT p.artist_id,
   FROM plays p
   LEFT JOIN liked l ON l.artist_id = p.artist_id
  ORDER BY score DESC, p.artist_id
- LIMIT 3;
+ LIMIT 5;
 
--- name: PickTopPlayedTrackForUser :one
--- For-You seed selection. Returns the user's most-played non-skipped
--- track in the last 7 days; tie-break by track_id for determinism.
+-- name: PickTopPlayedTracksForUser :many
+-- For-You candidate seeds. Returns the user's top-5 most-played
+-- non-skipped tracks in the last 7 days; tie-break by track_id for
+-- determinism. The Go-side picker (pickForYouSeedForDay) chooses one
+-- of the returned rows as today's seed via userIDHash so the
+-- candidate pool rotates day-to-day while staying stable within a
+-- day.
 SELECT t.id
   FROM play_events pe
   JOIN tracks t ON t.id = pe.track_id
@@ -80,7 +87,7 @@ SELECT t.id
    AND pe.was_skipped = false
  GROUP BY t.id
  ORDER BY COUNT(*) DESC, t.id
- LIMIT 1;
+ LIMIT 5;
 
 -- name: PickTopPlayedTrackForArtistByUser :one
 -- "Songs like X" seed selection. Returns the user's most-played non-skipped