feat: add authz validation to course optimizer tool

[dcoa]

Description

This PR updates the course optimizer endpoints with authz permissions systems. The frontend validates access to the tool with edit permissions but the actions are still using the previous authorization system.

With this change, roles such as course editors can run the Course Optimizer. All four endpoints check the openedx-authz courses.edit_course_content permission.

Testing instructions

Setup

1. Create three users (for example editor, auditor). None of them should be global staff or have legacy course roles.
2. Turn on the Course Optimizer for your test course. In Studio Django admin (/admin/waffle_utils/waffleflagcourseoverridemodel/), add a course override set to Force On for:
   • contentstore.enable_course_optimizer
   • contentstore.enable_course_optimizer_check_prev_run_links (only needed for scenario 4)
3. Turn on authz for the same course with a course override for authz.enable_course_authoring, set to Force On.
4. Assign authz roles in that course, either through the Admin Console or from a shell.

Scenario 1: a course editor can use the tool

1. Log in to the authoring MFE as editor and open the course.
2. Check the menu: under Tools, Course Optimizer is listed.
3. Start a scan: open it and click Start scan.
   • POST /api/contentstore/v0/link_check/<course_id> returns 200 {"LinkCheckStatus": "Pending"}.
   • Polling GET /api/contentstore/v0/link_check_status/<course_id> returns 200, and the scan results appear.
4. Compare with the old code: both calls returned 403.

Scenario 2: a course auditor is denied

1. Log in as auditor. Course Optimizer is not in the Tools menu.
2. Run GET <studio>/api/contentstore/v0/link_check_status/<course_id> and <studio>/api/contentstore/v0/rerun_link_update_status/<course_id>. Both return 403.

Scenario 3: behaviour is unchanged when authz is off

1. Delete the authz.enable_course_authoring override, or use a second course that has only the optimizer flag.
2. Add a user as Staff through the legacy Course Team page. They can run a scan and see the results, as before.
3. editor, who only has an authz role, now gets 403, because only the legacy roles are checked.

Scenario 4: previous-run link updates (optional)

This needs contentstore.enable_course_optimizer_check_prev_run_links and a rerun course whose content links to the original run.

1. As editor, scan the rerun. The results list links that point to the previous run.
2. Update one link: click Update on it. POST /api/contentourse_id> with {"action": "single", ...} returns 200.Polling rerun_link_update_status returns 200 with results.
3. Update all links: click Update all. {"action": "all"} returns 200.
4. As auditor, call the same endpoints. Both return 403.

[openedx-webhooks]

Thanks for the pull request, @dcoa!

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