feat: support role assignment for scopes that don't exist yet

[efortish]

Description

During course rerun, edx-platform needs to grant a user the instructor/staff role on the destination course key before that course's CourseOverview exists yet (the clone happens after role assignment would ideally occur). Today openedx-authz crashes in that case, so edx-platform (PR #38840) works around it with authz-specific branching that defers role assignment until after the course is cloned, plus a fallback check elsewhere in the meantime.

The actual crash: CourseScope.get_or_create_for_external_key() / ContentLibraryScope.get_or_create_for_external_key() call CourseOverview.get_from_id() / ContentLibrary.objects.get_by_key(), both of which raise DoesNotExist when the course/library isn't created yet.

This PR makes role assignment tolerant of that: the Scope row is created with its object FK left None, and gets linked up automatically once the real CourseOverview/ContentLibrary is saved.

Out of scope

Reverting the edx-platform PR #38840 workaround is explicit follow-up called out in the issue thread, not part of this change. The REST API's scope.exists() validation (rest_api/v1/serializers.py) is untouched — it's an intentional, separate guard for untrusted HTTP callers; this fix only affects the internal Python API path used directly by trusted platform code (e.g. student/roles.py's authz_add_role).

Solves #352

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