feat: report List-Unsubscribe urls and a differing Sender

A sweep of the user's real spam found List-Unsubscribe naming a domain
that appeared nowhere else in the message. It is attacker infrastructure
and was going unreported.

Every url from that header goes through redact.url() like a body url: an
unsubscribe link has to say who is unsubscribing, which makes it one of
the likeliest carriers of a recipient token. mailto: entries are skipped
rather than redacted, since the address is the whole value and nothing
useful survives removing it.

Sender is collected on the same terms as Reply-To, included only when it
differs from From. One repeating From is noise; one naming a separate
relay is the infrastructure behind the run.

Also drops the unused urlencode import left in redact.py when
_redact_kv_string stopped using urllib to rebuild the query string.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019NHaqA1Rz5ybed7wFUeQbK

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