[Feature] Affiliate code overhaul — universal 5% checkout code

## Overview
Remove individual creator shops. Replace with a universal affiliate code system. Every creator/affiliate gets a unique code that applies a 5% discount at checkout. Trackable and scalable.

## Scope
- Affiliate code generation system (admin-issued)
- Code applies 5% discount at checkout
- Code tracked to affiliate — sales attributed per code
- Affiliate reporting showing conversions and revenue attributed
- Codes can be deactivated by admin

## Acceptance criteria
- [ ] Admin can generate and issue unique affiliate codes
- [ ] Code applies 5% discount at checkout automatically
- [ ] Each sale attributed to the code used
- [ ] Affiliate reporting available to admin
- [ ] Codes can be activated and deactivated

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