feat: group abuse contacts into one destination per address

Two contacts can resolve to the same desk, an IP and a domain at one hoster
being the common case, and grouping per contact would send that desk two
mails about one incident.

The domain half of an address is folded to lowercase before grouping, since
it is case-insensitive by definition; the local part is left verbatim,
because only the receiving host knows whether it folds, and folding two
desks a host distinguishes would silently drop one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xj1ayFRSUQ2u7cwb3S4axE

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