Backlog: Discount and voucher logic

TL;DR

No discount or voucher engine exists today. Add later when marketing campaigns require it.

Description

• Voucher codes: percent, fixed amount, single-use vs multi-use, per-course vs global.
• Bulk codes generation for campaigns.
• Admin UI to manage codes.
• Reporting on redemption.

Acceptance Criteria

• t_vouchers table.
• Apply / validate at registration time.
• Audit redemptions per code.

Priority

• Priority: p3
• Rationale: not currently requested by client.

Dependencies

• None.

Links

• feat — registration