PATCH /workspaces/:id field-level auth for parent_id/tier/runtime
required a bearer token, blocking canvas nesting (drag-to-nest).
Added IsSameOriginCanvas check so the tenant canvas can update
sensitive fields without a bearer.
Exported IsSameOriginCanvas from middleware package so workspace.go
can call it for the field-level auth path.
DELETE /workspaces/:id is behind AdminAuth which already has the
same-origin check — if delete still fails, it's a different issue.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
3db589770e