refactor: use map[string]struct{} for allowedOrderBy whitelist

Per review feedback on PR #17, comment #137: use map[string]struct{}
(set semantics) instead of map[string]bool for the ORDER BY field
whitelist. Updates the lookup to comma-ok idiom.

internal/kurious/adapters/sqlite_course_repository.go

@@ -78,16 +78,16 @@ func (r *sqliteCourseRepository) List(
 		direction = "DESC"
 	}
 
-	allowedOrderBy := map[string]bool{
-		"id":         true,
-		"name":       true,
-		"created_at": true,
-		"updated_at": true,
-		"full_price": true,
-		"discount":   true,
-		"duration":   true,
+	allowedOrderBy := map[string]struct{}{
+		"id":         {},
+		"name":       {},
+		"created_at": {},
+		"updated_at": {},
+		"full_price": {},
+		"discount":   {},
+		"duration":   {},
 	}
-	if !allowedOrderBy[params.OrderBy] {
+	if _, ok := allowedOrderBy[params.OrderBy]; !ok {
 		return result, fmt.Errorf("invalid order by field: %s", params.OrderBy)
 	}