Account Ensure Key

The one way to get an API key: POST /api/account/signup, then this call. The first call mints the account's key and returns it once (created: true, key). Any later call never rotates or revokes: it returns has_key: true and the prefix of the key that stays valid (key is null, since keys are stored hashed). Bearer authentication is required; it consumes no trial or quota and has no billing side effect.