PlexConfig.acceptJson defaulted to true in both constructors and
travelled through copyWith, but no production or test caller ever passed
false, and every Plex path decodes JSON, so the header was conditional on
a flag that could not be off.
Emit `Accept: application/json` directly and drop the field. Headers on
the wire are unchanged.
Resolve platform, hardware model, and friendly device name once via a
shared DeviceIdentityService and send them to both backends: Plex gets
a real X-Plex-Platform plus X-Plex-Device/X-Plex-Device-Name (shown as
Player in dashboards/Tautulli), Jellyfin gets the device name in the
MediaBrowser auth header. Transcode and live-TV decision requests keep
their pinned platform names, which Plex validates server-side.
close#1270
edde746