Add license metadata

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

This changes only the README YAML frontmatter.

README.md

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