Start phone sign-in (send SMS code)

Passwordless phone sign-in, step 1: sends a 6-digit SMS one-time code (self-issued) to the given Australian mobile. Works for numbers with or without an existing account — verifying a new number creates the account (phone sign-in is also phone sign-up). Always returns 204; the response never reveals whether a number has an account. Re-sends are rate-limited per number and per IP.