Resend contract completion notification email

Resends the completion notification for a fully signed contract, honoring
the configured attachments. With no body, every participant with an email
address plus every configured extra recipient is notified. Pass
participant_ids and/or extra_emails to notify only a selection; an omitted
field means "everyone in that group" and an empty list means "nobody in
that group". extra_emails must be chosen from the extra recipients already
configured on this contract. This is a client dashboard Owner/Operations
action (403 if called via an API credential). Rate-limited to once per
minute per contract request (429 if called again too soon). Returns 404 if
the request is not found, 400 if the request is not completed, a selected
participant is unknown or has no email, an extra email is not configured,
or the selection resolves to no recipients.