Unlink Account

Unlink an account.

Removes the stored tokens (which is the part that grants Monni ongoing access to the external account) and every guild_verification row using it, so the link disappears from every server at once.

The account row itself is only hard-deleted when nothing references it as a ban subject. Where a ban does, the row is kept but scrubbed of display data and marked revoked: a cascade there would let anyone clear a ban record by unlinking, which is an obvious evasion route. What is retained in that case is the bare external id.