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

CLAUDE.md

@@ -212,6 +212,16 @@ cd /var/www/privapub.thepra.dev && sudo -u www-data ASPNETCORE_ENVIRONMENT=Produ
    sibling `media-proxy`, which `/media/files` does not serve.
 3. **A client never contacts a remote server for media:** every remote URL the API returns goes through
    `IMediaProxy.Wrap`, an HMAC-signed `/media/proxy/` URL fetched by `IFederationHttp.GetMedia`.
+4. **The proxy serves three ways:**
+   - **Cached:** a file already cached is served from disk, ranges included.
+   - **Downloaded:** a request without a `Range` is downloaded whole, up to `Media:MaxProxiedBytes`, then cached.
+   - **Streamed:** a ranged request, or anything too big to cache, is streamed from the origin with the range passed on,
+     and never cached. That is how remote video plays.
+
+   nginx has a `/media/proxy/` location with `proxy_buffering off` and a 600 s read timeout for those streams.
+5. **Remote video and audio become one playable attachment** in the Mastodon API (`MastodonMapper.Playable`): the best MP4
+   up to 720p that carries both sound and picture, including PeerTube's fragmented files inside an HLS entry. HLS
+   playlists themselves are not rewritten.
 
 ## Privacy invariants