Move the admin overview computation and struct into models.BuildOverview /
models.Overview, the admin create-user flow into models.CreateUserAsAdmin /
models.CreateUserBody, and the admin user response view into a new
pkg/routes/api/shared package (shared.AdminUser / shared.NewAdminUser) so both
the v1 and v2 admin routes call the same code. The v1 handlers are refactored
onto these helpers and stay byte-identical on the wire.
