Jellyfin 12 ships with EnableLegacyAuthorization=false, which drops the
legacy `api_key=` query spelling (jellyfin/jellyfin#15559). Every
authenticated URL Plezy self-authenticates via the query string then
fails with 401: trickplay sprite sheets (visible as missing scrub
thumbnails), transcoding/subtitle/Live TV URLs built through
_withApiKey, and the library-event websocket (403 on upgrade).
`ApiKey=` is read unconditionally by Jellyfin 10.8 through 12, while
Emby only accepts `api_key=`, so the parameter name now comes from
MediaBrowserDialect.tokenQueryParam. Emby output is byte-identical.
Image URLs keep `api_key`: Jellyfin serves item images without
authentication, and the artwork cache keys strip that exact name.
Fixes#2247