[Feature] Stats reset — pay to reset win/loss ratio

## Overview
Players can pay (in caps or real money — product decision) to reset their win/loss ratio. Caps sink mechanic with monetisation upside.

## Scope
- Stats reset purchasable via in-game shop (caps or plutonium — TBD)
- Resets: win count, loss count, win rate, ELO
- Reset logged internally (admin can see full history)
- Player-facing: stats show as fresh
- Cooldown period between resets to prevent abuse

## Acceptance criteria
- [ ] Reset purchasable via in-game shop
- [ ] Stats correctly zeroed on purchase
- [ ] Internal reset history logged (not visible to player)
- [ ] Cooldown enforced (suggested: 30 days)
- [ ] Reset cannot be used to manipulate active tournament standings

## Dependencies
- **Depends on:** OAuth, currency system