Get a presigned recording download URL

Returns a presigned S3 URL for the call's recording. Hand the URL straight to the customer or pull bytes server-side. The presigned URL is **time-limited** to 15 minutes; do not cache or embed it, request a fresh one each time you need the recording. Use `channel=mono` (default) for the composite recording (agent + caller mixed). Use `channel=dual` for a stereo recording (agent on one channel, caller on the other), which is available only when the call was captured with per-side audio. `?channel=dual` returns `404` when the dual URL is missing; retry with `?channel=mono` in that case. Errors: - `400 Invalid call ID format` if you pass a Mongo `_id` instead of the `callId` string. - `400 Invalid channel. Expected 'mono' or 'dual'` if `channel` is anything else. - `401 Unauthorized` if no API key or dashboard cookie is present. - `404 Call log not found` if the call is missing or belongs to another organization. (The literal `Call not found` string is only returned when the call itself is missing entirely; the util maps both to the same 404.) - `404 Dual-channel recording not found` if `channel=dual` was requested and only a mono capture exists.