feat(frontend): adapt to composite key transaction structure

- Update Transaction interface to include stable transaction_id field
- Modify TransactionsList to use stable transaction_id for React keys
- Update API models to handle new transactionId field from database
- Fix API routes to properly map transaction_id in responses
- Update test mocks to include transactionId field
- Ensure backward compatibility with internal_transaction_id

This adapts the frontend to work with the new composite primary key
(accountId, transactionId) structure that prevents duplicate transactions.

frontend/src/components/TransactionsList.tsx

@@ -267,10 +267,7 @@ export default function TransactionsList() {
 
             return (
               <div
-                key={
-                  transaction.internal_transaction_id ||
-                   `${transaction.account_id}-${transaction.transaction_date}-${transaction.transaction_value}`
-                }
+                 key={`${transaction.account_id}-${transaction.transaction_id}`}
                 className="p-6 hover:bg-gray-50 transition-colors"
               >
                 <div className="flex items-start justify-between">
@@ -365,7 +362,7 @@ export default function TransactionsList() {
         isOpen={showRawModal}
         onClose={handleCloseModal}
         rawTransaction={selectedTransaction?.raw_transaction}
-        transactionId={selectedTransaction?.internal_transaction_id || "unknown"}
+        transactionId={selectedTransaction?.transaction_id || "unknown"}
       />
     </div>
   );

frontend/src/types/api.ts

@@ -56,7 +56,8 @@ export interface RawTransactionData {
 }
 
 export interface Transaction {
-  internal_transaction_id: string | null;
+  transaction_id: string;  // NEW: stable bank-provided transaction ID
+  internal_transaction_id: string | null;  // OLD: unstable GoCardless ID
   account_id: string;
   transaction_value: number;
   transaction_currency: string;