Create unsubscribe API endpoints

[cka-y]

Description:
Implement two public (no auth required) endpoints for email-based unsubscription:

• GET /v1/subscriptions/{id} — look up a subscription by UUID, return info (type, status)
• DELETE /v1/subscriptions/{id} — deactivate subscription by UUID

For api.announcements, the DELETE handler must also remove the contact from the Brevo list. Also the api.announcements announcements should not be removed from the subcription, it should set active = false(this is to use the api.announcements as a token for email unsubscribe link).

Acceptance Criteria:

• GET /v1/subscriptions/{id} returns subscription info given a valid UUID
• DELETE /v1/subscriptions/{id} deactivates the subscription (sets active = false)
• Deactivation is idempotent (no-op on already-inactive subscription)
• For api.announcements, Brevo contact is removed from the list on deactivation
• UUID v4 provides sufficient security (no auth needed — only action is deactivation)

[jcpitre]

We will need a parameter for the DELETE for unsubscribe all, feed_updates or announcements.

[davidgamez]

As we are minimizing the information we are exposing in the endpoint, let's not implement GET endpoint