Create a new webhook subscription.

Creates a new webhook subscription for receiving real-time event notifications.

Available Events (assigned/working-partner role):

• case.assigned - Case validated by Debitura and assigned to your agency
• case.updated - Case lifecycle changed (e.g., Active → Paused). Does not cover engagement phase changes (Pre-legal/Legal/Enforcement) — poll GET /cases/{id} or GET /cases/{id}/timeline for the current phase; there is no phase-change event today.
• case.closed - Case closed
• payment.created - Payment registered on case
• payment.deleted - Payment reversed (deleted) on case
• chat.created - Chat message created on case

Security: Response includes Base64-encoded signing secret (shown only once). Store it to verify HMAC-SHA256 signatures on deliveries.

Requirements: HTTPS URL required. Deliveries retry with exponential backoff (8 attempts). Webhook auto-disables after max failures.