fix(server/m7-352): log Finish/Fail run errors; drop unused seed-limit constant

internal/playlists/system.go

@@ -76,10 +76,7 @@ func stableSortByScoreThenHash(cands []rankedCandidate, dateStr string) {
 	})
 }
 
-const (
-	systemMixLength    = 25
-	systemMixSeedLimit = 3
-)
+const systemMixLength = 25
 
 // systemMixWeights are the fixed scoring weights used by the cron worker.
 // JitterMagnitude is 0 because daily determinism comes from tieBreakHash;
@@ -126,17 +123,23 @@ func BuildSystemPlaylists(ctx context.Context, pool *pgxpool.Pool, logger *slog.
 	var buildErr error
 	defer func() {
 		if buildErr == nil {
-			_ = q.FinishSystemPlaylistRun(ctx, dbq.FinishSystemPlaylistRunParams{
+			if err := q.FinishSystemPlaylistRun(ctx, dbq.FinishSystemPlaylistRunParams{
 				UserID:      userID,
 				LastRunAt:   pgtype.Timestamptz{Time: now.UTC(), Valid: true},
 				LastRunDate: dateOnly,
-			})
+			}); err != nil {
+				logger.Warn("system playlist: failed to mark run finished; in_flight may be stuck",
+					"user_id", uuidStringPL(userID), "err", err)
+			}
 		} else {
 			errStr := buildErr.Error()
-			_ = q.FailSystemPlaylistRun(ctx, dbq.FailSystemPlaylistRunParams{
+			if err := q.FailSystemPlaylistRun(ctx, dbq.FailSystemPlaylistRunParams{
 				UserID:    userID,
 				LastError: &errStr,
-			})
+			}); err != nil {
+				logger.Warn("system playlist: failed to record build failure",
+					"user_id", uuidStringPL(userID), "err", err)
+			}
 		}
 	}()