Relay: a member's media through the server's buffer (proto 8)

Viewers who cannot reach the media get it through the server: a file on
any member's computer, a link passed on by a member ("Through me") or
fetched by the server ("Server fetches it", password-protected servers
only), and Jellyfin items for guests without an account, sourced by
whoever put them up with their own sign-in (or a pasted link's key),
which never leaves their machine. Fixes guests on a dedicated server
without a Jellyfin login being blocked.

The source dials /r/.../source (works behind NAT) and answers pulls with
an adaptive in-flight window (2 x rate x min RTT, 256 KiB..8 MiB). The
server keeps a memory-only buffer of 1 MiB slices sized like a viewer's
mpv buffer (pinned head/tail, 64 MiB back, 256 MiB ahead), fills it with
one sequential pull at a time (blocked viewers first, newest request
only), fetches nothing while nobody watches through it, and drops it on
the next swap or stop. Measured with mpv through shaped links
(tool/relay_e2e.dart): 400 Mbit/s seeks ~0.03 s; 40 Mbit/s no stalls,
3 viewers cost the source ~1x the bytes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/HOW-IT-WORKS.md

@@ -19,8 +19,11 @@ A five-minute tour. Details: [ARCHITECTURE.md](ARCHITECTURE.md), wire format:
  video bytes come from somewhere else:
    - a link (any http/https URL): each app fetches it directly
    - a Jellyfin item: each app plays it with its own Jellyfin account,
-     friends without one through the server (/j/... endpoint)
+     friends without one through the server
    - a file on the host's disk: the server streams it (/m/... endpoint)
+   - the relay (/r/...): a member's computer passes a file, a link or a
+     Jellyfin item on to the server, which keeps a few minutes in memory
+     and streams it to everyone who needs it
 ```
 
 - **App**: the desktop window. It draws the video with mpv, shows who is
@@ -31,7 +34,8 @@ A five-minute tour. Details: [ARCHITECTURE.md](ARCHITECTURE.md), wire format:
 - **Jellyfin** (or any web host): where the video lives. For a Jellyfin
   item the server only knows which item it is; each app signs in with its
   own account and reports what it watched. For friends without an account
-  the server fetches the item with the host's sign-in and passes it on.
+  the member who put it up passes it on through the server (their sign-in
+  stays on their computer).
 
 ## Ways to run