WorkspaceFileAssetEndpoint had no authorization checks beyond
authentication, allowing any logged-in user to create, read, patch,
and delete assets in any workspace by slug. DuplicateAssetEndpoint
only authorized the destination workspace, letting users copy assets
from workspaces they don't belong to.
Add @allow_permission decorators to all WorkspaceFileAssetEndpoint
methods and scope DuplicateAssetEndpoint's source asset lookup to
workspaces where the caller is an active member.
Ref: GHSA-qw87-v5w3-6vxx
