Renders a client-message template for a real client so staff can edit it in the composer before sending (ITE-1171). Values come from the tenant's records — client, and optionally the patient, appointment and estimate, each of which must belong to the client. Returns plain text for the channels the template claims, plus every {{Token}} left unfilled. Archived templates still render: a draft opened before the archive shouldn't break.

POST /templates/{id}/render (ITE-1171): render a client-message template for a real client so staff can edit it in the composer. Only the client is required; each other record adds the merge fields it can fill and must belong to that client.

A client-message template with the client's real values substituted. Plain text, values as stored — the composer shows it for editing, and the send path encodes it for the email's HTML. Only the channels the template claims are returned.