[Feature] Discount codes — admin-generated promotional codes

## Overview
Admin-generated discount codes applicable at Stripe checkout. Used for promotions, partnerships, events, and community giveaways.

## Scope
- Admin can generate single-use or multi-use codes
- Percentage or fixed-amount discount
- Expiry date
- Product/category restrictions optional
- Applied at Stripe checkout

## Acceptance criteria
- [ ] Admin can generate and manage codes
- [ ] Codes apply correctly at checkout via Stripe
- [ ] Expired and used codes rejected
- [ ] Usage tracked per code

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