[pierre-b]

Documents Notifuse/notifuse#509.

• features/broadcast-campaigns.mdx:
  • a new "Sender" section: the default sender, choosing one per variation, refusals, and what happens when a chosen sender is removed;
  • the "Change sender" dialog;
  • a "Sender per Variation" bullet in the A/B testing section.
• features/transactional-api.mdx: the email_options.sender_id row.
• openapi.json: BroadcastVariation.sender_id and email_options.sender_id, copied from the app's bundled spec.