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/playlists/system.go

@@ -294,7 +294,10 @@ func BuildSystemPlaylists(ctx context.Context, pool *pgxpool.Pool, logger *slog.
 	}
 
 	// 4. Discover: surface unheard tracks, biased toward dormant artists.
-	discoverTracks, derr := buildDiscoverCandidates(ctx, q, userID, dateStr)
+	// Individual bucket failures are logged inside buildDiscoverCandidates
+	// and redistribute as deficit; the function only returns an error for
+	// unrecoverable conditions, so a remaining derr here is genuine.
+	discoverTracks, derr := buildDiscoverCandidates(ctx, q, logger, userID, dateStr)
 	if derr != nil {
 		logger.Warn("system playlist: discover candidates load failed; skipping",
 			"user_id", uuidStringPL(userID), "err", derr)