Refresh broken ESRGAN 4x-UltraSharp download source

[cuken]

What to build

Refresh the ComfyUI ESRGAN downloader so the curated upscale model pull path works again or clearly marks the unavailable asset as optional.

During diagnosis, get_esrgan.sh failed on 4x-UltraSharp.pth with HTTP 401. The rest of the model pull sequence could continue, but this leaves the ESRGAN workflow path incomplete.

Acceptance criteria

• get_esrgan.sh no longer uses a URL that returns HTTP 401 for 4x-UltraSharp.pth.
• The script downloads 4x-UltraSharp.pth from a stable public source, or documents and skips it as unavailable/optional.
• RealESRGAN_x4plus.pth behavior is preserved.
• The script remains idempotent when files are already present.
• Failures identify the specific asset and source that failed.

Blocked by

None - can start immediately