Clear featured-list caches when featured learning paths are edited

[mbertrand]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/11979

Description (What does it do?)

Channel managers couldn't see featured-list edits for up to 24h (Redis view cache) plus 30min (Fastly page cache). Now:

• Editing a unit channel's featured learning path through the API (path update/delete, item add/reorder/remove, or the bulk add-to-list action) enqueues a post-commit celery task that clears the featured_resources Redis cache, hard-purges /c/unit/<name> in Fastly, and soft-purges the homepage.
• Nothing runs on the editor's request path; a failed enqueue (broker down) just logs.
• Frontend learning-path mutations also invalidate the featured React Query cache, so editors see the change without a full reload.
• call_fastly_purge_api gains a soft=True kwarg (sends Fastly-Soft-Purge: 1).

How can this be tested?

Run the full stack (COMPOSE_PROFILES=backend,frontend,keycloak,apisix) with the anonymous view cache enabled: set REDIS_VIEW_CACHE_DURATION=600 in env/backend.local.env (the shared default is 0, which disables it) and restart web + celery.

1. Seed featured lists, log in once at http://open.odl.local:8062 as the admin user (admin@odl.local / admin), then make that user a learning-path editor:

   docker compose run --rm web python manage.py populate_featured_lists
   docker compose exec -T web python manage.py shell -c "
   from django.contrib.auth import get_user_model
   from learning_resources.utils import update_editor_group
   update_editor_group(get_user_model().objects.get(email='admin@odl.local'), True)"

2. In a private/incognito window (anonymous), load a unit channel page — e.g. http://open.odl.local:8062/c/unit/mitx — and note the courses in the featured carousel. Load the homepage too. This primes the anonymous Redis cache for /api/v1/featured/.

3. In the window where you're logged in as the admin/editor user, go to http://open.odl.local:8062/learningpaths, open "MITx Featured Resources", and remove a course from the list (or add one via a course card's add-to-list dialog). You must be logged in as an admin or learning-path editor — anonymous or regular users can't see or edit learning paths. Watch the worker pick it up:

   docker compose logs -f celery | grep -iE "clear_featured|Skipping Fastly"

   Within a second or two of the edit you should see:

   Task learning_resources.tasks.clear_featured_caches[...] received
   Skipping Fastly purge for /c/unit/mitx (dev environment)
   Skipping Fastly purge for / (dev environment)
   Task learning_resources.tasks.clear_featured_caches[...] succeeded in 0.009s
   

   The Fastly purges are skipped locally (no FASTLY_API_KEY); on RC/prod they become real PURGE requests against /c/unit/<name> (hard) and / (soft).

4. Reload the incognito channel page and homepage: the featured carousel reflects the edit. Without this PR, the anonymous featured API response stays cached for REDIS_VIEW_CACHE_DURATION (24h default in deployed envs) and the pages keep rendering the old list.

I validated the backend chain locally by driving the same endpoints from the API side (editor DELETE on a featured list item → 204 → celery task ran in 9ms → anonymous /api/v1/featured/ fresh within seconds, where it stayed stale before). The React Query invalidation is covered by the jest tests (yarn test frontends/api/src/hooks/learningPaths frontends/api/src/hooks/learningResources).

Additional Context

• hq#11979 says "synchronously (best-effort, non-blocking on failure)"; this is asynchronous-after-commit instead. The task fires within a beat of the save, so the editor still sees changes within seconds, without in-request Redis/Fastly calls that could block or 504 the save.
• Fastly URL purges are exact-match, so query-string variants of the channel page (?topic=...) stay cached up to s-maxage (30 min). The base URL the editor refreshes is hard-purged. Per-channel surrogate keys are the upgrade path if that ever matters.
• Edits via Django admin/shell don't trigger invalidation — the hooks are on the viewsets deliberately, since model signals would fire on every ETL run.

[github-actions]

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