Fix audio2audio task type, demucs-web proxy, and health endpoint

- Map audio2audio to cover in Gradio args and Python spawn fallback
  (audio2audio is not a valid ACE-Step task type, cover is the equivalent)
- Add /demucs-web to Vite proxy so stem extractor opens correctly in Pinokio
  (in Pinokio, Vite runs on a dynamic port and /demucs-web was not proxied)
- Expose aceStepUrl in /api/generate/health response for debugging

vite.config.ts

@@ -25,6 +25,10 @@ export default defineConfig(({ mode }) => {
           target: 'http://127.0.0.1:3001',
           changeOrigin: true,
         },
+        '/demucs-web': {
+          target: 'http://127.0.0.1:3001',
+          changeOrigin: true,
+        },
       },
     },
     optimizeDeps: {