Delegate exchange persistence to ExchangeRepository

- Add create() and update() methods to ExchangeRepository
- Update ExchangeService to use repository methods instead of direct db operations
- All persistence operations now go through repositories consistently
- Fix indentation errors in ExchangeService

backend/repositories/exchange.py

@@ -161,3 +161,32 @@ class ExchangeRepository:
             )
         )
         return {row[0] for row in result.all()}
+
+    async def create(self, exchange: Exchange) -> Exchange:
+        """
+        Create a new exchange record.
+
+        Args:
+            exchange: Exchange instance to persist
+
+        Returns:
+            Created Exchange record (committed and refreshed)
+        """
+        self.db.add(exchange)
+        await self.db.commit()
+        await self.db.refresh(exchange)
+        return exchange
+
+    async def update(self, exchange: Exchange) -> Exchange:
+        """
+        Update an existing exchange record.
+
+        Args:
+            exchange: Exchange instance to update
+
+        Returns:
+            Updated Exchange record (committed and refreshed)
+        """
+        await self.db.commit()
+        await self.db.refresh(exchange)
+        return exchange