Add license metadata

Add model-card license metadata matching the primary upstream model, [Qwen/Qwen-Image-Layered](https://huggingface.co/Qwen/Qwen-Image-Layered).

This changes only the README YAML frontmatter.

README.md

@@ -1,4 +1,5 @@
 ---
+license: apache-2.0
 tags:
   - diffusion-single-file
   - comfyui