Add the media video and file shapes

Problem/Motivation

Video media and plain file fields are read on all three sites and produce two small, well-defined fragments — a URL with a playtime, and a URL with a size and a mime type. They are separate from images because the source field resolution and the metadata differ.

Proposed resolution

• media_video: resolve the media entity, return the URL and the playtime, handling both a file source and a remote video source.
• file: URL, size, mime type, and the original filename.
• Both return nothing rather than a stub when the reference is empty or the target is missing — a deleted file is a real case.
• Kernel coverage including the missing-target case.

Remaining tasks

Everything.