feat: quiz and rbac

internal/repository/stats_repo.go

@@ -39,7 +39,8 @@ type CategoryCount struct {
 // the table sizes are modest enough that chasing parallelism here
 // would buy sub-ms wins for added complexity.
 func (r *StatsRepo) Snapshot(ctx context.Context) (*StatsSnapshot, error) {
-	s := &StatsSnapshot{}
+	// Non-nil so an empty table serialises as [] rather than null.
+	s := &StatsSnapshot{FeedbackByCategory: []CategoryCount{}}
 	sevenAgo := time.Now().Add(-7 * 24 * time.Hour)
 
 	scalars := []struct {