Open a Kleinanzeigen session with the account's own credentials. Returns an opaque session id to carry in the x-session-id header on the session routes. The portal's own session is the only thing kept, sealed at rest; the password is not stored and cannot be read back out of the session. A Kleinanzeigen account is created in the Kleinanzeigen Android app, not through this API. Its password form is the one step behind a bot guard, so a plain HTTP client cannot post it: run that sign-in once in a browser, keep the refresh token it yields and send it as `login_token` instead of `password`. Everything after the login runs without a browser.