Add Emby as a third media server integration alongside Plex and Jellyfin.
Emby webhooks use a similar payload format (JSON with nested Item object
and ProviderIds). Also removes the mediaServerUsername field from all
webhook connections since it was never used for authentication — the
token-in-URL is the sole auth mechanism. This simplifies the connection
UX to a single "Connect" button.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
