Delegate price persistence to PriceRepository

- Add create() method to PriceRepository
- Update PriceService to use repository.create() instead of direct db operations

backend/services/price.py

@@ -52,12 +52,9 @@ class PriceService:
             price=price_value,
             timestamp=timestamp,
         )
-        self.db.add(record)
 
         try:
-            await self.db.commit()
-            await self.db.refresh(record)
-            return record
+            return await self.price_repo.create(record)
         except IntegrityError:
             # Duplicate timestamp - return the existing record
             await self.db.rollback()