Add live validation for model metadata hub slugs

## Summary

Add a live link check for every `hubSlug` in the website model metadata after the hub English model route supports variant aliases.

## Rationale

`apps/website/src/config/model-metadata.ts` maps local model IDs to Comfy Hub model pages. A unit test of an individual string literal cannot verify that the corresponding live hub page exists.

The English model route currently returns 404 for these valid variant aliases:

- `flux-1`
- `flux-1-kontext`
- `flux-2`
- `ltx-2`
- `ltx-2-3`
- `sdxl`

The locale route currently resolves aliases, but the English model route needs the workflow templates update tracked in Comfy-Org/workflow_templates#1164. The Hunyuan 3D entry requires the non-hyphenated hub slug `hunyuan3d`; `hunyuan-3d` returns 404.

## Affected area

- `apps/website/src/config/model-metadata.ts`
- External Comfy Hub workflow model routes

## Required changes

1. Wait for Comfy-Org/workflow_templates#1164, or otherwise confirm that the English hub route resolves supported aliases.
2. Add an automated live link check for all configured `hubSlug` values.
3. Make the check distinguish valid redirects from missing model pages.
4. Run the check in a suitable CI or scheduled environment that can access the live hub.

## Acceptance criteria

- Every configured `hubSlug` receives a live URL validation.
- Valid hub redirects pass the validation.
- Missing hub model pages fail the validation with the affected slug in the output.
- The check is enabled only after the external alias-resolution dependency is available.

## Backlinks

- Pull request: https://github.com/Comfy-Org/ComfyUI_frontend/pull/15500
- Review discussion: https://github.com/Comfy-Org/ComfyUI_frontend/pull/15500#discussion_r3824103855
- External dependency: Comfy-Org/workflow_templates#1164

Requested by: @mobeenabdullah