Get a time-limited recording download URL (legacy)

**Legacy.** Prefer [`GET /recordings/{callId}`](#get-a-presigned-recording-download-url) for new integrations. It supports both mono and dual channels via `?channel=mono|dual` and uses the same authenticated flow. Returns a presigned S3 URL for the call's mono composite recording. Hand the URL straight to the customer or pull bytes server-side. The presigned URL is time-limited to 15 minutes (same TTL as the new endpoint); do not cache it, request a fresh one each time you need the recording. Returns `404` if the call has no recording (call hasn't started, was cancelled before audio captured, or was deleted by the platform's retention policy). Returns `400 Invalid call ID format` if you pass a Mongo `_id` instead of the `callId` string.