Let super admins act as a user from the user detail dashboard for
support/debugging ("full with guardrails", audit-only).
Backend: AuthService.impersonate issues a short-lived token for the
target carrying an `act` claim (acting admin); refresh preserves it and
JwtStrategy surfaces `impersonatedBy`. Guardrails: cannot impersonate an
admin or a deactivated account; new ImpersonationGuard blocks sensitive
self-service (change own password/phone) while impersonating. New
AuditLog entity records impersonation start/stop (admin, target, ip,
time); admin endpoints POST users/:id/impersonate + .../impersonation/
stop and GET users/:id/audit.
Frontend: lib/impersonation swaps admin/impersonation tokens in
localStorage; persistent banner with exit; "Login as user" button and an
"Admin access log" tab on the detail page; logout clears impersonation.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
keyhan