Mark an activity as completed, or reopen a previously completed one. Body is optional; completed defaults to true. Reopening (completed: false) a non-custom activity (created automatically from a stage template) that is no longer on the deal's current stage is rejected with 400 — the internal service would otherwise soft-delete it instead of reopening it. Completions made through this endpoint carry no user attribution (completed_by stays null). Requires the ACTIVITIES_API feature flag and an API key with the activities:write scope.