DAR-147 — Add admin backoffice shared domain contracts
Introduce the admin bounded context to darvest-common: entities
(AdminUser, AuditLog, LoginLog, AccountStatusHistory), enums
(AdminRole, AccountStatus, AdminAction, AdminTargetType, LoginStatus),
and request/response DTOs for user management, role/status changes,
login log filtering, and wallet/user summaries.
Closes DAR-147