Set up announcement email template with custom unsubscribe link

[cka-y]

Description:
Update the announcement email template in Brevo to use a custom unsubscribe link in the footer instead of Brevo's native {{ unsubscribe }}. The link should use {{ contact.MDB_SUBSCRIPTION_ID }} to build the URL pointing to the frontend unsubscribe page.

Unsubscribe URL format:

https://mobilitydatabase.org/notifications/unsubscribe?id={{ contact.MDB_SUBSCRIPTION_ID }}

Reference: Successful feasibility test (template #68)

Acceptance Criteria:

• Announcement email template footer uses custom unsubscribe link with {{ contact.MDB_SUBSCRIPTION_ID }}
• Native Brevo {{ unsubscribe }} link is replaced
• Link correctly points to frontend unsubscribe page
• Tested with a test contact that has MDB_SUBSCRIPTION_ID set

[jcpitre]

• The link in the email should lead to a page allowing unsubscribe to everything related to the user in MDB_SUBSCRIPTION_ID:
  • Newsletter
  • Feed changes
  • all

Scope is only the email template.