P6: remote video and audio play through the proxy

- The media proxy streams ranged requests from the origin (passing the range on, never caching), downloads and caches
  whole files otherwise, and serves cached files with range support. A PeerTube video is never fetched whole for one
  viewer, and clients still never contact the remote host.
- PeerTube's fragmented MP4 files inside an HLS entry are read as variants, so HLS-only instances play too.
- A remote Video or Audio post becomes one playable Mastodon attachment: the best MP4 up to 720p that carries both
  sound and picture, with its poster and duration; the card is kept only when nothing is playable.
- nginx: /media/proxy/ with proxy_buffering off and a 600 s read timeout (applied on the box, with a backup).

Checked live: a GoToSocial image through the proxy answers 206 with exactly the asked range when streamed, 200 when
cached, and 206 with the right Content-Range from the cache.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Federation/Objects/ObjectShapes.cs

@@ -217,10 +217,13 @@ namespace PrivaPub.Federation.Objects
 			if (Value(note, "type") != "Video")
 				return default;
 			var links = Objects(note["url"]).ToList();
+			var nested = links.Where(l => Value(l, "mediaType")?.Equals("application/x-mpegURL", StringComparison.OrdinalIgnoreCase) == true)
+				.SelectMany(l => Objects(l["tag"]))
+				.ToList();
 			return new VideoDetails
 			{
 				DurationSeconds = Seconds(note["duration"]),
-				Variants = Variants(links, "video/", "audio/"),
+				Variants = Variants(links.Concat(nested).ToList(), "video/", "audio/"),
 				PlaylistURL = links.Where(l => Value(l, "mediaType")?.Equals("application/x-mpegURL", StringComparison.OrdinalIgnoreCase) == true)
 					.Select(l => Value(l, "href")).FirstOrDefault(IsWeb),
 				IsLive = Bool(note, "isLiveBroadcast") == true,