feat: Locker management, package tracking, and referral program backend modules

[demilade18-git]

Summary

This PR implements four features assigned to Demilade18-git:

[BE-18] Locker Management: Backend (#1182)

• New Locker entity: lockerNumber (unique), floor, size (SMALL/MEDIUM/LARGE), assignedToUserId (nullable FK), assignedAt, isActive, notes, soft-delete support
• POST /lockers — create locker (admin only)
• PATCH /lockers/:id — update locker details/status (admin only)
• DELETE /lockers/:id — soft-delete (admin only; only if unassigned)
• GET /lockers — list all lockers (admin/staff)
• GET /lockers/mine — returns member's assigned locker or null
• POST /lockers/:id/assign — assign locker to member (admin only)
• POST /lockers/:id/unassign — release assignment (admin only)
• LockersModule registered in AppModule

[FE-16] Locker Management: Frontend (#1183)

• /lockers page showing prominent locker card (large locker number) when assigned, or "No locker assigned" message with Request a Locker CTA
• useGetMyLocker React Query hook calling GET /lockers/mine
• /lockers link added to DashboardSidebar navigation

[BE-16] Referral Program: Backend (#1184)

• GET /referrals/my-code — returns current user's referral code, shareable link URL, and total referrals count
• GET /referrals/history — paginated list of successful referrals (referee name, join date)
• ReferralsModule registered in AppModule

[BE-17] Package / Mail Tracking: Backend (#1185)

• New Package entity: recipientUserId, loggedByStaffId, courierName, trackingNumber, description, arrivedAt, collectedAt, status (ARRIVED/COLLECTED/RETURNED)
• POST /packages — log package arrival (staff/admin only)
• PATCH /packages/:id/collect — mark as collected (staff/admin)
• PATCH /packages/:id/return — mark as returned to sender (staff/admin)
• GET /packages/mine — member views their own packages
• GET /packages — admin/staff views all packages (filterable by status)
• PackagesModule registered in AppModule

Closes #1182
Closes #1183
Closes #1184
Closes #1185

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