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/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)
 				}