Email-link CLI login works as a polled device flow

A terminal can log in by email link alone, with no dashboard and no copy-paste, using a device-flow shape. The CLI POSTs the email and receives a private ticket, the server emails the link, and the CLI polls every couple of seconds. When the user confirms in the browser, the server marks the ticket approved and returns the minted API key to the next poll holding that ticket, once only. Keep the ticket out of the emailed link so a forwarded email cannot retrieve the key. Non-TTY installs need a path that does not hang on the prompt or the poll.