Return a tenant-scoped normalized call and SMS timeline for one customer (calls:read AND sms:read).

Identify the customer with any one of customer_id, lead_id, phone, email or external_id — the same resolver /v1/customers uses. Items are normalized (source_type/source_id, conversation_id, customer_id/lead_id, provider/provider_id, occurred_at) while source preserves the original record, so a consumer can act without knowing which internal table held the artifact. Paging is an opaque keyset over the immutable created_at across both channels; walk it with next_cursor. updated_since matches a call on its updated_at and an SMS on its created_at — SMS rows carry no updated_at, so a delivery-status change on an already-synced message is NOT visible here; use the event/delivery endpoints for that.