Send a verification code to an address

Starts the signup that ends in an account key, with no key and no login. Sends a 6 digit code to the address, valid for 10 minutes and 5 attempts, and answers 202 with the verify call. Sends are capped at 3 per address per hour, 10 per IP per hour, and 25 per client fingerprint per day; past a cap the response is 429 signup_send_limited with retry_after_seconds and an unlock whose action is this call. An address that already has an account gets a code too; verifying it mints a key on that account. Send { "email": "agent@example.com" }, with an optional "src" naming the surface that sent you.