All admin API error responses (and the global error handler) now emit
RFC 7807 application/problem+json bodies (type/title/status/detail)
instead of the ad hoc { error: string } shape, per review feedback on
this PR. Scoped to admin.ts + the global handler in index.ts, since
that's what this PR already touches; public.ts's routes still use the
old shape pending a follow-up.
59eb54d9fb