FAC-84 feat: add super admin user directory endpoint

[y4nder]

Summary

• add a super-admin-only GET /admin/users endpoint for the future admin console
• support paginated filtering by search, role, active status, and campus/department/program scope
• add response DTOs, OpenAPI query examples, and focused controller/service tests

Verification

• npm test -- --runInBand src/modules/admin/admin.controller.spec.ts src/modules/admin/services/admin.service.spec.ts
• npm run build

Closes #187