feat: 新增交易服务和AI记账服务，实现交易的创建、验证及账户余额更新逻辑。

internal/service/ai_bookkeeping_service.go

@@ -462,8 +462,9 @@ func (s *LLMService) ParseIntent(ctx context.Context, text string, history []Cha
 1. 金额：提取数字，如"6元"=6，"十五"=15
 2. 分类：根据内容推断，如"奶茶/咖啡/吃饭"=餐饮，"打车/地铁"=交通，"买衣服"=购物
 3. 类型：默认expense(支出)，除非明确说"收入/工资/奖金/红包"
-4. 日期：默认使用今天的日期（%s），除非用户明确指定其他日期
+4. 金额：提取明确的数字。如果用户未提及具体金额（如只说"想吃炸鸡"），amount字段必须返回 0
-5. 备注：提取关键描述
+5. 日期：默认使用今天的日期（%s），除非用户明确指定其他日期
+6. 备注：提取关键描述
 
 直接返回JSON，不要解释：
 {"amount":数字,"category":"分类","type":"expense或income","note":"备注","date":"YYYY-MM-DD","message":"简短确认"}
@@ -1257,7 +1258,7 @@ func (s *AIBookkeepingService) getDefaultCategory(userID uint, txType string) (*
 // getMissingFields returns list of missing required fields
 func (s *AIBookkeepingService) getMissingFields(params *AITransactionParams) []string {
 	var missing []string
-	if params.Amount == nil {
+	if params.Amount == nil || *params.Amount <= 0 {
 		missing = append(missing, "amount")
 	}
 	if params.CategoryID == nil && params.Category == "" {

internal/service/transaction_service.go

@@ -351,6 +351,9 @@ func (s *TransactionService) UpdateTransaction(userID, id uint, input Transactio
 
 		// Step 1: Reverse the old transaction's effect on balances
 		oldReversedBalance := calculateNewBalance(oldAccount.Balance, existingTxn.Amount, existingTxn.Type, false)
+		if !oldAccount.IsCredit && oldReversedBalance < 0 {
+			return fmt.Errorf("%w: update would cause negative balance in account '%s'", ErrInsufficientBalance, oldAccount.Name)
+		}
 		if err := txAccountRepo.UpdateBalance(userID, existingTxn.AccountID, oldReversedBalance); err != nil {
 			return fmt.Errorf("failed to reverse old account balance: %w", err)
 		}
@@ -358,6 +361,9 @@ func (s *TransactionService) UpdateTransaction(userID, id uint, input Transactio
 		// Reverse old transfer destination if applicable
 		if oldToAccount != nil {
 			oldToReversedBalance := oldToAccount.Balance - existingTxn.Amount
+			if !oldToAccount.IsCredit && oldToReversedBalance < 0 {
+				return fmt.Errorf("%w: update would cause negative balance in old destination account '%s'", ErrInsufficientBalance, oldToAccount.Name)
+			}
 			if err := txAccountRepo.UpdateBalance(userID, *existingTxn.ToAccountID, oldToReversedBalance); err != nil {
 				return fmt.Errorf("failed to reverse old destination account balance: %w", err)
 			}
@@ -449,6 +455,9 @@ func (s *TransactionService) DeleteTransaction(userID, id uint) error {
 
 		// Reverse the transaction's effect on balance
 		reversedBalance := calculateNewBalance(account.Balance, existingTxn.Amount, existingTxn.Type, false)
+		if !account.IsCredit && reversedBalance < 0 {
+			return fmt.Errorf("%w: deletion would cause negative balance in account '%s'", ErrInsufficientBalance, account.Name)
+		}
 		if err := txAccountRepo.UpdateBalance(userID, existingTxn.AccountID, reversedBalance); err != nil {
 			return fmt.Errorf("failed to reverse account balance: %w", err)
 		}
@@ -461,6 +470,9 @@ func (s *TransactionService) DeleteTransaction(userID, id uint) error {
 			}
 			if toAccount != nil {
 				reversedToBalance := toAccount.Balance - existingTxn.Amount
+				if !toAccount.IsCredit && reversedToBalance < 0 {
+					return fmt.Errorf("%w: deletion would cause negative balance in destination account '%s'", ErrInsufficientBalance, toAccount.Name)
+				}
 				if err := txAccountRepo.UpdateBalance(userID, *existingTxn.ToAccountID, reversedToBalance); err != nil {
 					return fmt.Errorf("failed to reverse destination account balance: %w", err)
 				}