feat: retire legacy Hunyuan video support

[atomantic]

Summary

• retire the legacy HunyuanVideo MLX model, installer, runtime, and render helper
• migrate existing shipped registry entries to FastMetal while preserving user-repointed records
• fail closed on historical Hunyuan runtime records so they cannot reach an incompatible fallback renderer

Test plan

• full server Vitest suite: 35,569 passed, 38 skipped
• focused retirement/catalog/runtime tests: 307 passed
• bash syntax validation for scripts/setup-image-video.sh
• git diff --check