[BE-55] Membership plan management: backend (rebuild deleted module)

[yusuftomilola]

Overview

The membership-plans module (backend/src/membership-plans/) was removed from the working tree. Membership tiers are a core revenue driver — without plan management, every booking is one-off and the hub loses recurring subscription revenue.

Context

• The original module entities (MembershipPlan, UserMembership) and enums exist in git history but are deleted — rebuild
• PaymentsModule for Paystack subscription payments
• User entity: add activeMembershipId FK (nullable)
• EmailModule for subscription confirmation and cancellation emails

Tasks

• Create MembershipPlan entity: id, name, description (text), priceKobo (int), billingCycle (MONTHLY/QUARTERLY/YEARLY), features (JSONB string array, e.g. ["Unlimited hot-desk access", "5 meeting room hours/month"]), bookingHoursIncluded (int, 0 = unlimited), guestPassesPerMonth (int, default 0), isActive (bool), displayOrder (int), createdAt, updatedAt
• Create UserMembership entity: id, userId (FK), planId (FK), status (ACTIVE/CANCELLED/EXPIRED/PAST_DUE), startDate, currentPeriodEnd, cancelledAt (nullable), paystackSubscriptionCode (nullable), createdAt
• GET /membership-plans — list active plans sorted by displayOrder (public, no auth)
• GET /membership-plans/:id — plan detail (public)
• POST /membership-plans — create plan (admin only)
• PATCH /membership-plans/:id — update plan (admin only); cannot change priceKobo if plan has active subscribers
• POST /membership-plans/:id/subscribe — member subscribes to a plan; initialises Paystack recurring charge; creates UserMembership with ACTIVE status; sends welcome email
• DELETE /membership-plans/my-subscription — cancel subscription; sets cancelledAt; membership remains ACTIVE until currentPeriodEnd; sends cancellation email
• GET /membership-plans/my-subscription — get current user's active subscription with plan details
• Register MembershipPlansModule in AppModule

Files to Modify / Create

• New: backend/src/membership-plans/ (module, entities, controller, providers)
• backend/src/app.module.ts