feat: build promo codes and discount backend module

[soundsng]

Summary

• Adds a full PromoCodesModule with PromoCode and PromoCodeUsage entities, DTOs, service, and controller
• Implements promo code validation (expiry, max uses, minimum booking amount, workspace type restrictions, per-user idempotency) and atomic usage recording via a database transaction
• Wires the module into the payments webhook flow so usage is recorded automatically on successful charge

Changes

• backend/src/promo-codes/ — new module (entities, DTOs, service, controller, module)
• backend/src/bookings/entities/booking.entity.ts — added appliedPromoCodeId and promoDiscountApplied nullable columns
• backend/src/payments/providers/handle-webhook.provider.ts — calls recordUsage on successful payment if a promo code was applied
• backend/src/payments/payments.module.ts — imports PromoCodesModule
• backend/src/app.module.ts — imports PromoCodesModule

closes #1084

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[yusuftomilola]

Very unique way of approaching problems.