feat(api): add currency extraction and account name updates

- Extract currency from balances and populate account currency field
- Add PUT /api/v1/accounts/{account_id} endpoint for updating account names
- Add AccountUpdate Pydantic model for request validation
- Modify sync service to enrich account details with balance currency

This resolves the issue where account currency and name fields were NULL
by extracting currency from GoCardless balance data and providing an API
endpoint for manual account name updates.

leggend/api/models/accounts.py

@@ -33,6 +33,15 @@ class AccountDetails(BaseModel):
         json_encoders = {datetime: lambda v: v.isoformat() if v else None}
 
 
+class AccountUpdate(BaseModel):
+    """Account update model"""
+
+    name: Optional[str] = None
+
+    class Config:
+        json_encoders = {datetime: lambda v: v.isoformat() if v else None}
+
+
 class Transaction(BaseModel):
     """Transaction model"""