fix: use BiRefNet-Lite as default model and fix JSON parsing

- Switch default from birefnet-general (973MB, 4min) to
  birefnet-general-lite (faster, still SOTA quality)
- Fix Python script stdout pollution — progress messages now go to
  stderr so the JSON result parser doesn't break
- Pre-download birefnet-general-lite in Docker build

apps/web/src/components/tools/remove-bg-settings.tsx

@@ -34,7 +34,7 @@ export function RemoveBgSettings() {
   const { processFiles, processing, error, downloadUrl, originalSize, processedSize } =
     useToolProcessor("remove-background");
 
-  const [model, setModel] = useState<BgModel>("birefnet-general");
+  const [model, setModel] = useState<BgModel>("birefnet-general-lite");
   const [bgColor, setBgColor] = useState("");
   const [elapsed, setElapsed] = useState(0);
   const timerRef = useRef<ReturnType<typeof setInterval> | null>(null);