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

docs/ROADMAP.md

@@ -465,11 +465,13 @@ it, raw where it doesn't.
   - otherwise the server reads the page (owner decision 1): public posts only, 0–60 s after arrival, cached per address
     for 7 days across the whole server, `Federation:FetchLinkPreviews` to switch it off. Checked live against a
     GoToSocial profile page.
-- **Media:**
-  - video playback through the proxy (Range requests, HLS playlist rewriting, the poster), with a `video` card;
+- **Media** (done in v1.14.0 except where noted):
+  - video playback through the proxy: range requests streamed from the origin, the poster, a playable attachment picked
+    from MP4 or fragmented MP4 files (HLS playlists are not rewritten; not needed while PeerTube publishes the files);
   - audio attachments;
-  - an article reader view;
-  - JPEG/PNG renditions kept for Pixelfed.
+  - JPEG/PNG for Pixelfed: uploads were already re-encoded to JPEG, PNG or GIF;
+  - an article reader view: the data is in `Status.privapub` (title, excerpt, cover, full HTML in `content`); the view
+    itself belongs to the client.
 
 #### P7 Threads, communities and the social graph
 - **Thread backfill:**