Explicit registration of an email that already has an account is rejected before a signup code is issued. The conflict is account_exists. A code verified after that account exists is consumed and does not issue credentials or change the owner. Legacy code sign-in remains available. The mobile sign-in and recovery choices are app #863 (f8bdd8c0fe161b81460eb92601bc4f899e1fadd3).

Merged into backend staging in #591 as 045319d3. Not deployed. Local gates only.