Delete every render a user requested (native app, admin offboarding).

Native app only (DVT-4925): 404 unless the render app-stage is configured. Admin only (render:delete; 403 otherwise). Hard-deletes up to limit (default 100, max 500) of the named user's non-thumbnail, non-in-flight renders in one transaction, one audit row per render. In-flight renders are skipped. more: true means further renders remain: call again. The response is NOT wrapped in the {data, meta} envelope. A failure after some objects were already removed is 500 and is recorded as dashboard.render.delete.commit_failed; retrying converges.