fix(playlists): unblock Discover by removing SELECT DISTINCT + ORDER BY plan error

The cross-user bucket query combined SELECT DISTINCT with ORDER BY md5(...),
which Postgres rejects at plan time (SQLSTATE 42P10). buildDiscoverCandidates
returned that error on first bucket failure, so the whole Discover playlist
was skipped every nightly run — even though the random bucket pool was
healthy. Switched to GROUP BY so the md5 ordering expression no longer needs
to appear in the select list, and hardened the function so future single-
bucket failures degrade gracefully via slot redistribution instead of taking
out the whole playlist.

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

internal/db/dbq/discover.sql.go

@@ -12,7 +12,7 @@ import (
 )
 
 const listCrossUserLikedTracksForDiscover = `-- name: ListCrossUserLikedTracksForDiscover :many
-SELECT DISTINCT t.id, t.album_id, t.artist_id
+SELECT t.id, t.album_id, t.artist_id
   FROM general_likes gl
   JOIN tracks t ON t.id = gl.track_id
  WHERE gl.user_id != $1
@@ -30,6 +30,7 @@ SELECT DISTINCT t.id, t.album_id, t.artist_id
        SELECT 1 FROM lidarr_quarantine q
         WHERE q.user_id = $1 AND q.track_id = t.id
    )
+ GROUP BY t.id, t.album_id, t.artist_id
  ORDER BY md5(t.id::text || $2::text)
  LIMIT 60
 `
@@ -50,6 +51,11 @@ type ListCrossUserLikedTracksForDiscoverRow struct {
 // rows (the caller's slot redistribution rolls the deficit into the
 // other two buckets).
 // $1 = user_id, $2 = date string for md5 ordering.
+//
+// GROUP BY (not SELECT DISTINCT) so the md5 ORDER BY expression need
+// not appear in the select list — Postgres rejects DISTINCT + ORDER BY
+// by-expression at plan time (SQLSTATE 42P10), which previously caused
+// the entire Discover build to fail silently.
 func (q *Queries) ListCrossUserLikedTracksForDiscover(ctx context.Context, arg ListCrossUserLikedTracksForDiscoverParams) ([]ListCrossUserLikedTracksForDiscoverRow, error) {
 	rows, err := q.db.Query(ctx, listCrossUserLikedTracksForDiscover, arg.UserID, arg.Column2)
 	if err != nil {

internal/db/queries/discover.sql

@@ -52,7 +52,12 @@ SELECT t.id, t.album_id, t.artist_id
 -- rows (the caller's slot redistribution rolls the deficit into the
 -- other two buckets).
 -- $1 = user_id, $2 = date string for md5 ordering.
-SELECT DISTINCT t.id, t.album_id, t.artist_id
+--
+-- GROUP BY (not SELECT DISTINCT) so the md5 ORDER BY expression need
+-- not appear in the select list — Postgres rejects DISTINCT + ORDER BY
+-- by-expression at plan time (SQLSTATE 42P10), which previously caused
+-- the entire Discover build to fail silently.
+SELECT t.id, t.album_id, t.artist_id
   FROM general_likes gl
   JOIN tracks t ON t.id = gl.track_id
  WHERE gl.user_id != $1
@@ -70,6 +75,7 @@ SELECT DISTINCT t.id, t.album_id, t.artist_id
        SELECT 1 FROM lidarr_quarantine q
         WHERE q.user_id = $1 AND q.track_id = t.id
    )
+ GROUP BY t.id, t.album_id, t.artist_id
  ORDER BY md5(t.id::text || $2::text)
  LIMIT 60;