[Feature] Refer a player — referral mechanic for subscribers and purchasers

## Overview
Existing subscribers or purchasers can refer new players via a unique referral link. Reward triggered on qualifying action.

## Scope
- Unique referral link per player
- New player signs up via referral link
- Reward triggered on qualifying action (first purchase or subscription start)
- Referrer rewarded (caps, discount code, or cosmetic — product decision)
- Attribution tracked correctly

## Acceptance criteria
- [ ] Referral link generated per player
- [ ] Attribution tracked from link click to qualifying action
- [ ] Reward automatically issued to referrer on qualifying action
- [ ] Referral history visible on profile

## Dependencies
- **Depends on:** OAuth, Stripe