Hono middleware that forwards /api/* to ARENA_UPSTREAM_URL when set, with
standalone mode (env unset) and /api/health untouched. Verified end-to-end
against a real second HTTP server: register a bot upstream, read it back
through the proxy, method/query/JSON body forwarded unchanged, inbound Host
header dropped, response content-encoding/content-length stripped.
TDD: 6 tests written first and confirmed failing (module didn't exist),
then implemented to green.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
