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

nanovllm-voxcpm uses a single asyncio event loop internally
(loop.run_until_complete). With concurrency > 1 multiple Gradio
worker threads hit the same loop simultaneously, causing
"RuntimeError: This event loop is already running" on every request.

Made-with: Cursor

README.md

@@ -61,5 +61,5 @@ Recommended environment variables:
 - `NANOVLLM_TEMPERATURE`: defaults to `1.0`
 - `REQUEST_LOG_DIR`: optional persistent request log directory. Defaults to `/data/logs` when `/data` exists
 - `GRADIO_QUEUE_MAX_SIZE`: defaults to `10`
-- `GRADIO_DEFAULT_CONCURRENCY_LIMIT`: defaults to `1`
+- `GRADIO_DEFAULT_CONCURRENCY_LIMIT`: defaults to `1` (nanovllm-voxcpm event loop is not thread-safe; do NOT increase)
 - `GRADIO_SSR_MODE`: defaults to `false`