Get unified timeline for a customer

Paginated, unified activity feed — inbound/outbound calls, SMS, notes, tasks, appointments, file uploads, custom field changes — sorted newest-first. This endpoint is read-only; to create activity, hit the specific endpoints (/notes, /schedule-call, /send-sms, etc.) and the timeline entry is written automatically. Aggregation is capped at ~1000 activities per contact; use limit/offset pagination for very active contacts to avoid slow loads.