NVIDIA encoder chroma corruption fixed; per-publication encoder selection added

Fixes

• Fix NVIDIA encoder I420 uploads to copy each plane using its actual source stride, avoiding chroma corruption when source frames use padded YUV planes. Also fix the local_video publisher reusing mutable I420 frame storage after handing frames to WebRTC.
• Add per-publication video encoder backend selection. Add a video encoder backend availability query. Remove LIVEKIT_PREFERRED_HW_ENCODER in favor of per-publication backend selection.