Backend:
- Add migrations/075_create_webhooks.sql: webhooks + webhook_failures tables
- Fixes GET /webhooks 500 (relation "webhooks" did not exist)
Frontend:
- Skip toast for 5xx on /webhooks so developer portal shows empty state
instead of 'Une erreur serveur s'est produite' when table is missing
Co-authored-by: Cursor <cursoragent@cursor.com>
