Add provider asset backend metadata and remote asset environment ownership support

video_style currently has no shared concept of whether a provider's derivatives are local or remote, which makes it hard for sibling provider modules to apply consistent safety rules around shared remote accounts. This follow-up should extend the provider definition with explicit asset backend metadata, add shared environment-normalization and ownership-guard infrastructure, and expose the resulting blocked state in the shared asset admin/status UX. The goal is to keep cross-provider policy in video_style while letting provider submodules remain thin integrations.