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/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