_on_invite only auto-joins a room when the inviter is allow-listed (or
GATEWAY_ALLOW_ALL_USERS is set), so a live invite from an arbitrary
federated user is rejected. A pending invite that arrives while the
gateway is down takes a different path: _schedule_pending_invite_joins
reconciles it from rooms.invite in the sync response and scheduled the
join unconditionally. An unauthorized invite sent during downtime was
therefore auto-joined on restart, bypassing the allowlist.
Extract the gate from _on_invite into _is_authorized_inviter and apply
it during reconciliation too, reading the inviter from the stripped
invite state (the sender of the m.room.member event for our own user,
as _extract_invite_dm_signal already does for the DM signal). An
inviter that cannot be read from the invite state fails closed, exactly
like an empty sender in _on_invite: the invite is skipped with a
warning and left pending.
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