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/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;