Reverse geocode a point

Degrades to a divisions-only result (source: "divisions_only", plus a
note) if the addresses theme is unreachable, missing, or has no nearby
coverage — addresses is Overture's newest, least complete theme, so
this is the expected degraded path. Returns a structured {"error": ...}
instead of raising if the remote scan fails outright.

The answer also carries top-level "country" (ISO 3166-1 alpha-2) and
"region" (ISO 3166-2) beside admin_context, from the same nearest
division the chain is built from — omitted, not null-filled, when the
dataset or division lacks a value.