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.

leggend/api/models/accounts.py

@@ -36,7 +36,8 @@ class AccountDetails(BaseModel):
 class Transaction(BaseModel):
     """Transaction model"""
 
-    internal_transaction_id: Optional[str] = None
+    transaction_id: str  # NEW: stable bank-provided transaction ID
+    internal_transaction_id: Optional[str] = None  # OLD: unstable GoCardless ID
     institution_id: str
     iban: Optional[str] = None
     account_id: str
@@ -54,6 +55,7 @@ class Transaction(BaseModel):
 class TransactionSummary(BaseModel):
     """Transaction summary for lists"""
 
+    transaction_id: str  # NEW: stable bank-provided transaction ID
     internal_transaction_id: Optional[str] = None
     date: datetime
     description: str