Fix Jellyfin 12 authorization

[mayvqt]

Summary

• send Jellyfin tokens through the supported Authorization: MediaBrowser scheme
• preserve Emby's existing provider-specific token behavior
• document Jellyfin 12 compatibility

Validation

• go test ./internal/integrations/mediaserver
• git diff --check
• go test ./internal/http/handlers and go test ./... are blocked by the pre-existing TestAdminIntegrationsRendersUsefulStatusCards DNS stall against the fake arr host