feat: 添加预算管理功能，包括其handler、service和repository层。

internal/repository/budget_repository.go

@@ -66,8 +66,19 @@ func (r *BudgetRepository) Update(budget *models.Budget) error {
 		return fmt.Errorf("failed to check budget existence: %w", err)
 	}
 
-	// Update the budget
-	if err := r.db.Save(budget).Error; err != nil {
+	// Update the budget using Select to explicitly specify fields to update
+	// This ensures that nil/NULL values for CategoryID and AccountID are properly saved
+	if err := r.db.Model(budget).Select(
+		"name",
+		"amount",
+		"period_type",
+		"category_id", // Explicitly include to allow setting to NULL
+		"account_id",  // Explicitly include to allow setting to NULL
+		"is_rolling",
+		"start_date",
+		"end_date",
+		"updated_at",
+	).Updates(budget).Error; err != nil {
 		return fmt.Errorf("failed to update budget: %w", err)
 	}
 	return nil