Replaced the internal DevJpegCoverArt flag with a user-facing
EnableWebPEncoding config option (defaults to true). When disabled, the
fallback encoding now preserves the original image format — PNG sources
stay PNG for non-square resizes, matching v0.60.3 behavior. The previous
implementation incorrectly re-encoded PNG sources as JPEG in non-square
mode. Also added EnableWebPEncoding to the insights data.