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

leggend/api/routes/accounts.py

@@ -243,7 +243,8 @@ async def get_account_transactions(
             # Return simplified transaction summaries
             data = [
                 TransactionSummary(
-                    internal_transaction_id=txn["internalTransactionId"],
+                    transaction_id=txn["transactionId"],  # NEW: stable bank-provided ID
+                    internal_transaction_id=txn.get("internalTransactionId"),
                     date=txn["transactionDate"],
                     description=txn["description"],
                     amount=txn["transactionValue"],
@@ -257,7 +258,8 @@ async def get_account_transactions(
             # Return full transaction details
             data = [
                 Transaction(
-                    internal_transaction_id=txn["internalTransactionId"],
+                    transaction_id=txn["transactionId"],  # NEW: stable bank-provided ID
+                    internal_transaction_id=txn.get("internalTransactionId"),
                     institution_id=txn["institutionId"],
                     iban=txn["iban"],
                     account_id=txn["accountId"],

leggend/api/routes/transactions.py

@@ -75,7 +75,8 @@ async def get_all_transactions(
             # Return simplified transaction summaries
             data = [
                 TransactionSummary(
-                    internal_transaction_id=txn["internalTransactionId"],
+                    transaction_id=txn["transactionId"],  # NEW: stable bank-provided ID
+                    internal_transaction_id=txn.get("internalTransactionId"),
                     date=txn["transactionDate"],
                     description=txn["description"],
                     amount=txn["transactionValue"],
@@ -89,7 +90,8 @@ async def get_all_transactions(
             # Return full transaction details
             data = [
                 Transaction(
-                    internal_transaction_id=txn["internalTransactionId"],
+                    transaction_id=txn["transactionId"],  # NEW: stable bank-provided ID
+                    internal_transaction_id=txn.get("internalTransactionId"),
                     institution_id=txn["institutionId"],
                     iban=txn["iban"],
                     account_id=txn["accountId"],