Function resolve_video_download_url 

pub async fn resolve_video_download_url(
    repo: &dyn MediaRepository,
    item_id: &str,
    quality: &str,
    media_source_id: Option<&str>,
) -> String

Resolve the download URL for a video, applying the audio-codec policy that keeps the saved file playable offline (DR-171).

Every video download goes through here rather than calling the builder directly: the builder is pure and cannot look the codec up, and a caller that forgets to is exactly how the silent downloads shipped.

TRACES: UR-071 | DR-171