Jellyfin refs on the wire + guest relay (proto 7)

- Media can be a Jellyfin ref {serverId, itemId, mediaSourceId, title,
  subtitle, runtimeSecs}; the server never stores or broadcasts a URL
  or token for it.
- Optional server-side Jellyfin credential (serve map / --jellyfin
  file) enables GET /j/<sid>/<token>/<itemId>: range-faithful proxy of
  the active item only, exact client header fields, upstream aborted on
  disconnect, bytes counted. welcome.relayUrl / state.relay advertise it.
- Members report jf access (own account vs relay).
- /m now sends the real Content-Type (Relic overrode it).

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

packages/server/lib/src/media_endpoint.dart

@@ -136,7 +136,9 @@ void mountMediaRoute(RelicApp app, File file,
     log(byteCount);
     return Response(
       res.status,
-      body: Body.fromDataStream(stream, contentLength: byteCount),
+      // Relic replaces Content-Type with the body's mime type, so set it here.
+      body: Body.fromDataStream(stream,
+          mimeType: _mimeOf(res.headers['content-type']), contentLength: byteCount),
       headers: Headers.build((h) {
         res.headers.forEach((k, v) => h[k] = [v]);
       }),
@@ -208,10 +210,20 @@ void mountSessionMediaRoute(
     });
     return Response(
       res.status,
-      body: Body.fromDataStream(stream, contentLength: byteCount),
+      // Relic replaces Content-Type with the body's mime type, so set it here.
+      body: Body.fromDataStream(stream,
+          mimeType: _mimeOf(res.headers['content-type']), contentLength: byteCount),
       headers: Headers.build((h) {
         res.headers.forEach((k, v) => h[k] = [v]);
       }),
     );
   });
 }
+
+MimeType _mimeOf(String? contentType) {
+  try {
+    return MimeType.parse(contentType ?? '');
+  } catch (_) {
+    return MimeType.octetStream;
+  }
+}