fix: harden async nanovllm demo stability

fix: revert concurrency limit to 1 — nanovllm event loop is not thread-safe

fix: store daily request logs on persistent disk

fix: add denoiser and reference audio guards

fix: harden Gradio NanoVLLM startup path

fix: switch Gradio demo backend to nanovllm_voxcpm

upgrade to VoxCPM2 + Gradio 6 with full i18n UI

Update README.md

initial commit