SendCodeAsync validates the invite, then starts the code with a conditional
update that also requires the invite to still be open. When an admin revoked
the link (or it was used) between those two reads, the refusal was reported as
resend_too_soon with a Retry-After, although the link was already dead.
On a refused start the invite is now read again: a closed invite gets the same
generic invalid_invite response as any other dead link, and a cooldown or send
limit refusal is computed from the fresh row.