Issues a one-time token and returns a t.me/<bot>?start=<token> deep link. The user opens it in Telegram and presses Start; the bot webhook then completes the link.
t.me/<bot>?start=<token>
Deep link and token expiry
deep_link
expires_at
Link status (none | pending | linked)
status
telegram_username
linked_at
Telegram unlinked
