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>

packages/server/lib/src/media_endpoint.dart

@@ -249,3 +249,23 @@ MimeType _mimeOf(String? contentType) {
     return MimeType.octetStream;
   }
 }
+
+/// Says `connection: close` on a response to a request that asked for it
+/// (ffmpeg always does). dart:io closes such a connection after the
+/// response but, behind relic, without saying so; ffmpeg (8.x) then sends
+/// its next seek on that dead connection after reading a range to its end
+/// (an mp4's tail) and the seek fails for good ("size -38").
+Handler closeWhenAsked(Handler next) => (req) async {
+      final res = await next(req);
+      if (res is! Response) return res;
+      bool asked;
+      try {
+        asked = req.headers.connection?.isClose ?? false;
+      } catch (_) {
+        asked = false;
+      }
+      if (!asked) return res;
+      return res.copyWith(
+          headers: res.headers.transform(
+              (h) => h.connection = const ConnectionHeader.directives([ConnectionHeaderType.close])));
+    };