Create backend admin dashboard endpoints

[yusuftomilola]

Description:
Add admin-only endpoints to the Dashboard controller: GET /dashboard/admin/stats (system-wide stats + monthly registration trend + newsletter stats) and GET /dashboard/admin/users (paginated user list with search). Both require admin role.

Acceptance Criteria:

• GET /dashboard/admin/stats — returns user counts, monthly trend, newsletter stats
• GET /dashboard/admin/users — paginated, searchable user list
• Both endpoints protected with @Roles(UserRole.ADMIN) and RolesGuard
• Search supports filtering by name or email
• Pagination with page and limit query params

File(s): backend/src/dashboard/dashboard.controller.ts, backend/src/dashboard/dashboard.service.ts

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[wumibals]

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