bug: fix reward calculation on edge cases

[diiabblo]

Description

Incorrect reward amounts calculated when user answers exactly at timeout.

Steps to Reproduce

1. Start a game
2. Answer final question right as timer hits 0
3. Observe reward amount

Expected Behavior

Consistent reward calculation regardless of timing.

Actual Behavior

Sometimes shows 0 reward or double reward.

Possible Solution

Add timestamp validation and race condition protection.