Relay: viewers keep the file size (headers within 4 s, connection: close), retries back off, 412 re-resolves

Two server behaviours left ffmpeg (8.1/9.0) without the file size, after
which every seek failed with "size -38" until a reload:
- dart:io holds response headers until body bytes exist: a stalled
  source meant no headers within mpv's 5 s timeout. Relay byte responses
  are now written on the raw connection, headers with the first bytes or
  after 4 s, and a swap ends waiting reads with a plain close.
- every response closed its socket without saying connection: close, so
  ffmpeg 8.x reused a dead connection for its next seek (pol's X clips).
  The server now says it whenever the request asks.
A failed pull waits 250 ms doubling to 8 s instead of re-pulling at once
(the ~130 ms storm that drew Bilibili's 412/403); 412 counts as an expired
URL and re-resolves the page, at most once per 60 s. The app reloads a
player once per media when ffmpeg reports the lost size anyway. P37: a
read that waits past ffmpeg's retry budget still ends the file early.

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

docs/ROADMAP.md

@@ -58,6 +58,14 @@ Problems are numbered; delete a line when its fix ships.
 - P36 After a reconnect the app re-dials the warm relay of an Up next
   item that was already played ("relay channel refused: not the source of
   Up next item q8"): harmless, the feeder should end with its item.
+- P37 A relay viewer whose read waits on bytes the source does not send
+  for longer than ffmpeg's reconnect budget (4 tries per read, about
+  40 s with the 4 s header rule) gets an I/O error that mpv takes as the
+  end of the file. Measured with mpv 0.41 / ffmpeg 9.0.2 and the source
+  away about a minute across a seek: it played on; one read stalled past
+  the budget ends the file. Options: a longer `reconnect_delay_max` for
+  relay URLs only, or the end-of-file path reopening a relay viewer
+  that stopped short while the server plays.
 
 **Restructure** (in order; reviewed against the code). Truth lives in
 three places: the server (members, presence, gate, transport, queue incl.