Stamp one onboarding milestone for the caller (spec 1.2).

<br/>{"event": "link_copied"} is the only body this accepts; anything else<br/>is 422 invalid_event. The portal calls it when the affiliate copies<br/>their link, which is the one checklist step with no server seam of its<br/>own: copying happens in the clipboard, not in an API call.<br/><br/>Idempotent by construction. The first stamp wins in SQL, so two tabs<br/>pressing copy cannot move a date the checklist has already shown, and the<br/>route answers 204 either way rather than telling the client whether it<br/>was first.