Ed25519 challenge (app principal)
Fetches a Stellar Ed25519 challenge for the given app public key.
The gateway proxies GET /auth-connect?account=<G...> to the Zeam
anchor. The caller signs the returned challenge with their Stellar
seed and submits it to POST /v1/public/auth-connect.
GET
Ed25519 challenge (app principal)
Ed25519 challenge (app principal)

