Devise 4.9.x has a race condition in the reconfirmable flow where
concurrent email change requests can desynchronize the confirmation
token from `unconfirmed_email`, letting an attacker confirm an email
they don't own. We use `:confirmable` with `reconfirmable = true`, so
we're directly exposed.
The upstream fix is in Devise 5.0.3, but we can't upgrade —
`devise-two-factor` only supports Devise 5 from v6.4.0, which also
raised its Rails minimum to 7.2+. No released version supports both
Devise 5 and Rails 7.1.
This PR ports the Devise 5.0.3 fix locally by overriding
`postpone_email_change_until_confirmation_and_regenerate_confirmation_token`
on the User model to persist the record before regenerating the token.
This is a stopgap — remove it once the dependency chain allows upgrading
to Devise 5.
### How to test
Sign in as a confirmed user and change your email. The app should send a
confirmation to the new address while keeping the current email
unchanged until confirmed.
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