- Add comprehensive promotion code methods to Order model
- Implement Stripe invoice integration for promotion code discounts
- Display promotion codes on invoice with proper discount breakdown
- Fix and enhance all unit tests for promotion code functionality
- Add discount calculation with capping to prevent negative totals
- Ensure promotion codes work across entire order lifecycle
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
