feat(newsletter): add List-Unsubscribe headers and RFC 8058 one-click handler

[Neophytis]

Summary

• Mailer::sendNewsletterEmail(): now emits three bulk-mail headers on every regular newsletter send:
  • Precedence: bulk — standard signal for mass mail
  • List-Unsubscribe: <https://...> — personalised per recipient using the existing unsubscribe token (required by Google/Yahoo since Feb 2024 for senders over 5,000/day)
  • List-Unsubscribe-Post: List-Unsubscribe=One-Click — enables Gmail's one-click unsubscribe button
• Transactional sends (reminder, suspension notification, Terms of Use) are excluded from bulk headers.
• SubscriptionController::unsubscribeNewsletter(): handles RFC 8058 one-click POST requests (body List-Unsubscribe=One-Click) sent by Gmail when a user clicks the unsubscribe button, immediately suppressing the address with a 200 response and no confirmation page.

Implementation notes

sendTemplateEmail() gains an array $extraTextHeaders = [] parameter (backward-compatible default). A new private buildNewsletterHeaders() method constructs the header map, returning empty for transactional types or when no unsubscribe key is present.

The unsubscribe URL is generated via the existing regular_newsletter_unsubscribe route with the per-recipient unsubscribe_key already passed through SendMassmailCommand.

Test plan

• Send a test newsletter to a personal address; confirm List-Unsubscribe, List-Unsubscribe-Post, and Precedence appear in raw headers
• Verify the unsubscribe URL in the header resolves to the correct member's unsubscribe page
• POST to the unsubscribe URL with body List-Unsubscribe=One-Click; confirm 200 and that the member is unsubscribed
• Send a reminder/suspension notification; confirm none of the bulk headers appear