Newsletter: subscriptions with double opt-in, campaigns, one-click unsubscribe and bounces

Arnel, 14 September 2026: a newsletter site is one of the systems a barako expert should be able to brew. The pieces exist or are filed (content, email templates #829, schedule trigger #839, email events in barakoBrew, contacts #250), except the parts that make it a newsletter and keep it legal.

## Change

A `BarakoCMS.Newsletter` module:
- **Subscriptions.** A public subscribe endpoint with double opt-in (a confirmation email with a signed link), stored per list, rate limited, honeypot like Forms (#720).
- **Unsubscribe.** A signed one-click link and the `List-Unsubscribe` and `List-Unsubscribe-Post` headers, honoured immediately and never re-subscribed by an import.
- **Campaigns.** A campaign is an entry: a template (#829), a list, a segment filter, and a send time (#839). Sending goes through the Email modules in batches with the provider's limits, one record per recipient.
- **Events back.** Bounces and complaints from the provider suppress the address; opens and clicks are optional and off by default.
- **Consent record.** When and how each address subscribed, exportable for a data request.

## Done when

- Subscribe, confirm, receive a scheduled campaign, and unsubscribe in one click, with a suppressed address never sent to again, covered by tests against a fake provider.

## Where it lives

Module: `BarakoCMS.Newsletter`, owning subscriptions, campaigns and consent records, and sending through the core's email provider interface.

Start with the proposed BarakoCMS.Newsletter module and the core email provider interface, then review email templates #829, schedule trigger #839, contacts #250, and Forms #720. Use a fake provider to cover subscription, confirmation, scheduled campaign delivery, one-click unsubscribe, suppression, consent records, and the stated headers and rate limits.

Written by the indexing model from the issue text.