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.

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)