Add license metadata

Add model-card license metadata matching the primary upstream model, [Wan-AI/Wan2.1-T2V-14B](https://huggingface.co/Wan-AI/Wan2.1-T2V-14B).

This changes only the README YAML frontmatter.

README.md

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