Poll finalize: decide plus-one handling for voters with a standing RSVP

[leahpeker]

Context

While scoping PR #1272 (Issue 1238 — poll finalize bypasses RSVP capacity), plus-one handling was pulled out as a separate, lower-priority concern.

Polls carry no +1 concept — a voter doesn't express a +1 when casting their vote. _seat_voter in backend/community/_poll_finalize.py currently seats a winning voter using whatever has_plus_one value already exists on their standing EventRSVP row, if one exists:

# Polls carry no +1 concept; only an existing row's standing +1 survives.
has_plus_one = existing is not None and existing.has_plus_one

This is an edge case (a voter who already had an RSVP with a +1 before the poll ran) rather than the common path, but the intended behavior hasn't been decided — should their standing +1 carry through to a seat won by poll, should it be dropped, or should something else happen?

What needs to happen

Decide and implement the intended behavior for a poll voter who has an existing RSVP with has_plus_one=True at the time the poll they voted in is finalized.

Acceptance criteria

• TODO — decide whether a standing +1 should carry through poll-seating, be dropped, or be handled some other way, then implement accordingly.