fix: resolve GraphQL Int overflow on large historical volumes

- Change the olume type from int to loat in StockHistoryType and StockTickType in schema.py to prevent 32-bit signed integer serialization limits from throwing errors on large volume assets (e.g. BTC-USD max range).

backend/app/graphql/schema.py

@@ -58,13 +58,13 @@ class StockHistoryType:
     high: float
     low: float
     close: float
-    volume: int
+    volume: float
 
 @strawberry.type
 class StockTickType:
     ticker: str
     price: float
-    volume: int
+    volume: float
     timestamp: str
 
 @strawberry.type