feat: add seed_sandbox_data management command

[efortish]

What this does

Closes #382. Right now testing course-authoring permissions on the Sandbox means manually creating orgs, users, and role assignments before you can even start. This adds a seed_sandbox_data management command that does that setup in one shot, and is safe to rerun whenever you need a fresh environment.

• Fixture: openedx_authz/management/commands/data/sandbox_seed_data.json ships a default org and 6 users, each with one role/scope pair covering the main course-authoring and library roles (platform-wide, org-wide, course-level, library-level).
• Command: python manage.py cms seed_sandbox_data reads that fixture, creates the org/users with get_or_create, assigns roles through the real assign_role_to_user_in_scope API, then reloads the Casbin policy. Running it again just skips what's already there instead of duplicating it.
• --reset deletes the fixture's users first, for a clean slate.
• --data-file /path/to/other.json swaps in a different fixture without touching the bundled one.
• Every org/user/role-assignment failure is logged individually and counted. The command prints a created/skipped/failed summary and raises CommandError if anything failed, instead of finishing green with half the data missing.

Two things I left out on purpose. First, creating real course/library content: the fixture just points at ids that should already exist in the target Sandbox. Since #369 made role assignment work even when the scoped object doesn't exist yet, pointing at a not-yet-created course id won't error either, but it won't be much use for clicking around Studio until that course actually shows up. Second, wiring this into a CI/CD step (the optional part of the issue's acceptance criteria), that belongs to the Sandbox deploy pipeline, not this repo.

How to test this manually

1. Get a shell in a CMS process running this branch (in an eduNEXT devstack, that's pip install -e /path/to/openedx-authz inside the container after checking out the branch).
2. Run the seeder:

   python manage.py cms seed_sandbox_data
   

   On a fresh environment you should see Seeding complete: 7 created, 0 skipped, 0 failed. (the org plus the 6 users). If you'd already seeded before, the ones that exist show up as skipped instead.
3. Check what landed:

   python manage.py cms shell -c "
   from django.contrib.auth import get_user_model
   from openedx_authz.api.users import get_user_role_assignments_filtered
   for u in get_user_model().objects.filter(username__startswith='authz_'):
       for a in get_user_role_assignments_filtered(user_external_key=u.username):
           for role in a.roles:
               print(u.username, '->', role.external_key, '@', a.scope.external_key)
   "
   

   You should see all 6 users with their expected role and scope.
4. Run the command again with no flags. It should report everything as skipped, not create duplicates.
5. Run python manage.py cms seed_sandbox_data --reset. The 6 users get deleted and recreated (their DB id will change if you check before/after).
6. To actually log into Studio/LMS as one of these users (password edx by default) and verify what that role can and can't do: check that the course/library ids in the fixture match something real in your Sandbox first. The bundled default (course-v1:OpenedX+DemoX+DemoCourse) is just whatever demo course I had imported locally, it's very likely not the one in your environment.

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