Delegate price persistence to PriceRepository

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

backend/repositories/price.py

@@ -56,3 +56,18 @@ class PriceRepository:
             )
         )
         return result.scalar_one_or_none()
+
+    async def create(self, record: PriceHistory) -> PriceHistory:
+        """
+        Create a new price history record.
+
+        Args:
+            record: PriceHistory instance to persist
+
+        Returns:
+            Created PriceHistory record (refreshed from database)
+        """
+        self.db.add(record)
+        await self.db.commit()
+        await self.db.refresh(record)
+        return record