feat(referrals): backend referral system with codes, tracking, rewards

[Icahbod]

Closes #1091
Closes #1101
Closes #1092
Closes #1093

Implements the referral system backend module (codes, tracking, rewards).

What it adds

• New User.referralCode column (varchar, nullable, unique).
• Referral entity with ReferralStatus (PENDING / COMPLETED) and RewardType (DISCOUNT / CREDIT) enums.
• ReferralsService:
  • ensureReferralCode — generates an 8-hex-char MH-XXXXXXXX code and retries up to 5 times on unique-constraint collision.
  • getMyCode — returns the code plus a shareable URL, configurable via FRONTEND_URL with APP_URL and https://managehub.vercel.app fallbacks.
  • getStats — totals, conversions, total rewards earned.
  • getAll — admin-only list of every referral.
  • createReferral(referralCode, referredUserId) — idempotent on referredUserId.
  • completeReferral(referredUserId) — marks the first PENDING referral for the user as COMPLETED and defaults the reward to 10% DISCOUNT.
• ReferralsController exposes GET /referrals/my-code, GET /referrals/stats, and GET /referrals (admin / super_admin only).
• CreateUserDto accepts an optional referralCode (max 64 chars).
• AuthService.createUser records the relationship when ?ref=CODE is supplied at signup, using NestJS Logger for failure visibility (a referral-tracking failure never blocks registration).
• HandleWebhookProvider.handleChargeSuccess calls completeReferral after the booking is confirmed — fire-and-forget so payment processing is never blocked.
• 9 Jest unit tests for ReferralsService (all green on this branch).

Out of scope

Production ALTER TABLE users ADD COLUMN referral_code varchar UNIQUE migrations — dev runs with synchronize: true, so no migration is generated. Please run the equivalent migration manually when promoting to staging / production.

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[yusuftomilola]

Thanks for your contribution