fix: Prevent user from cancelling trades after slot time has passed

Users can no longer cancel trades once the slot time has passed.
Added test to verify this behavior.

backend/routes/exchange.py

@@ -548,7 +548,13 @@ async def cancel_my_trade(
             detail=f"Cannot cancel: status is '{exchange.status.value}'",
         )
 
-    # Cancel the exchange (no time restriction per spec)
+    # Check if slot time has already passed
+    if exchange.slot_start <= datetime.now(UTC):
+        raise HTTPException(
+            status_code=400,
+            detail="Cannot cancel: trade slot time has already passed",
+        )
+
     exchange.status = ExchangeStatus.CANCELLED_BY_USER
     exchange.cancelled_at = datetime.now(UTC)