[GF-BE-09] Build promo codes and discount backend module

[yusuftomilola]

Problem

There is no promo code or discount system on ManageHub. Admins cannot create promotional codes for onboarding campaigns, corporate rates, or seasonal discounts. Members have no way to apply a discount at checkout, which limits the platform's marketing flexibility.

Proposed Solution

Create a new NestJS module at backend/src/promo-codes/.

Entity: PromoCode

• id (UUID), code (string, unique — stored uppercase)
• discountType (enum: percentage | fixed_amount)
• discountValue (integer — percentage 1–100, or fixed kobo amount)
• maxUses (integer, nullable — null = unlimited)
• usedCount (integer, default 0)
• minBookingAmount (integer, kobo — minimum booking value to qualify, default 0)
• applicableWorkspaceTypes (array of WorkspaceType enum, nullable — null = all types)
• expiresAt (timestamp, nullable)
• isActive (boolean, default true)
• createdAt, updatedAt

Entity: PromoCodeUsage

• id (UUID), promoCodeId (FK), userId (FK), bookingId (FK)
• discountApplied (integer, kobo), usedAt (timestamp)

Endpoints:

Method	Path	Access
POST	/promo-codes	Admin
GET	/promo-codes	Admin
PATCH	/promo-codes/:id	Admin
DELETE	/promo-codes/:id	Admin
POST	/promo-codes/validate	Authenticated member

POST /promo-codes/validate accepts { code, workspaceId, bookingAmount } and returns { valid: boolean, discountType, discountValue, finalAmount, message }. The payment service then reads this when initialising a booking payment.

Acceptance Criteria

• Both entities created with migrations
• code is normalised to uppercase on save
• POST /promo-codes/validate checks: code exists, isActive, not expired, under maxUses, workspace type matches, booking amount meets minBookingAmount
• A user cannot use the same promo code more than once (enforced via PromoCodeUsage unique constraint on promoCodeId + userId)
• usedCount increments atomically when a payment using the code succeeds (hook into handle-webhook.provider.ts)
• PromoCodesModule is registered in AppModule

[soundsng]

Hey maintainer. I have a proven track record and already understand the codebase and how to implement this issue without obstacles.

[grantfox-oss]

🦊 GrantFox — @soundsng has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #1084)
2. Your PR will be reviewed by the DistinctCodes maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@soundsng's PR #1144 was approved and merged by @yusuftomilola.

🏆 @soundsng: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (296 total points). Track your full progress on GrantFox.

👏 Great work, @soundsng! Keep contributing to DistinctCodes.