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/Infrastructure/Http/FederationHttp.cs

@@ -23,6 +23,7 @@ namespace PrivaPub.Infrastructure.Http
 		Task<FetchedJson> GetJson(string url, string accept, Action<HttpRequestMessage> sign, CancellationToken token);
 		bool FailedTemporarily(string url);
 		Task<(Uri FinalUri, string Html)> GetPage(string url, CancellationToken token);
+		Task<HttpResponseMessage> OpenMedia(string url, System.Net.Http.Headers.RangeHeaderValue range, CancellationToken token);
 		Task<HttpResponseMessage> Send(HttpRequestMessage request, CancellationToken token);
 		Task<(byte[] Bytes, string ContentType)> GetMedia(string url, long maxBytes, CancellationToken token);
 	}
@@ -229,6 +230,46 @@ namespace PrivaPub.Infrastructure.Http
 			}
 		}
 
+		public async Task<HttpResponseMessage> OpenMedia(string url, System.Net.Http.Headers.RangeHeaderValue range, CancellationToken token)
+		{
+			if (!Uri.TryCreate(url, UriKind.Absolute, out var target) || !IsAllowed(target))
+				return default;
+			try
+			{
+				for (var hop = 0; hop <= MaxRedirects; hop++)
+				{
+					using var request = new HttpRequestMessage(HttpMethod.Get, target);
+					request.Headers.Accept.ParseAdd("video/*, audio/*, image/*");
+					request.Headers.Range = range;
+					var response = await _httpClientFactory.CreateClient(ClientName).SendAsync(request, HttpCompletionOption.ResponseHeadersRead, token);
+					if (IsRedirect(response.StatusCode))
+					{
+						var location = response.Headers.Location;
+						response.Dispose();
+						var next = location == default ? default : location.IsAbsoluteUri ? location : new Uri(target, location);
+						if (!IsAllowed(next))
+							return default;
+						target = next;
+						continue;
+					}
+					var mediaType = response.Content.Headers.ContentType?.MediaType?.ToLowerInvariant();
+					if (!response.IsSuccessStatusCode || mediaType == default || mediaType.Contains("svg")
+						|| !(mediaType.StartsWith("video/") || mediaType.StartsWith("audio/") || mediaType.StartsWith("image/") || mediaType == "application/octet-stream"))
+					{
+						response.Dispose();
+						return default;
+					}
+					return response;
+				}
+				return default;
+			}
+			catch (Exception ex) when (ex is HttpRequestException or BlockedDestinationException or OperationCanceledException && !token.IsCancellationRequested)
+			{
+				_logger.LogInformation("Media stream {Url} refused: {Reason}", url, ex.Message);
+				return default;
+			}
+		}
+
 		const int MaxPageBytes = 512 * 1024;
 
 		static async Task<byte[]> ReadPrefix(HttpContent content, int limit, CancellationToken token)