Fallback sync task for announcements

[cka-y]

Description:
Create a scheduled task that periodically polls the Brevo contact list and reconciles any drift with the database. This serves as a safety net in case the real-time sync on subscribe/unsubscribe misses an update.

Can reuse get_contact_subscription_status() from brevo.py. Low priority if the sync on subscribe/unsubscribe is reliable.

Acceptance Criteria:

• Scheduled task runs on a configurable interval
• Compares Brevo contact list with notification_subscription rows for api.announcements
• Reconciles differences (adds missing subs, deactivates removed ones)
• Logs reconciliation actions
• Reuses existing brevo.py helper functions