Behavior: Generates and returns an MRSS feed in XML format. The XML is serialized without the XML declaration and uses the media namespace (http://search.yahoo.com/mrss/) for media-specific elements such as media:description. Returns 404 if the feed model cannot be generated.

Authorization: Requires an authenticated user (class-level [Authorize]).

Side Effects: None. This is a read-only operation.

Error Scenarios: Returns 401 if the user is not authenticated. Returns 404 if the feed data is not available.