Get Contact Timeline

The funnel timeline for one contact: what was done, what it cost, what came back.

Read-only. Tenancy: the key's own organisation is the only schema read, and the contact must be visible to the key (id_kind="pk", the namespace the timeline sources are keyed on, so the gate authorises the same row the handler serves). Every step kind is present in a fixed order; count and cost_credits are null when not recorded, which is not the same as 0. Returns 404 for a contact outside the key's organisation or visibility and 504 when the read times out.