Replaces the old X-API-Token-based RequireAdmin in middleware.go with a
context-aware RequireAdmin() that runs after RequireUser, checks
user.IsAdmin, and returns 403 {"error":"not_authorized"} for non-admins
or 500 {"error":"internal_error"} if RequireUser was bypassed. Updates
server.go to mount RequireUser then RequireAdmin on the /api/admin group.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
