[Spoiler Alert!] feat: Add Rating sample

[kdmccormick]

Backend:

• UnitRating (per-user 1-5 star rating of a unit; uses opaque_keys UsageKeyField) and CourseAverageRating (cached per-course aggregate kept in sync incrementally on writes, fully recomputed on full course publish).
• DRF endpoints at /api/v1/unit-rating/ and /api/v1/course-average-rating/.
• Second filter pipeline step on Learner Home /init injects averageStars and ratingCount onto each courseRun, alongside the existing isArchivedByLearner.
• XBLOCK_PUBLISHED handler recomputes only when block_type == "course" so unit/section publishes don't re-aggregate.
• Admin registrations for both new models.

Frontend:

• RateThisContent: per-unit star widget for frontend-app-learning's sequence_container.v1 slot.
• CourseCardRating: per-course average display, embedded inside the Archive CourseList card (rather than the platform course-card slot). Renders "(N ratings)" even at 0 so the wiring is visibly working before any ratings exist.

Tutor plugin:

• Second PLUGIN_SLOTS entry registers RateThisContent in the learning MFE, next to the existing learner-dashboard course_list slot.

Frontend README:

• env.config.jsx instructions rewritten around a single shared file that covers both slots and gets copied into each MFE root.

A handful of @@TODOs remain in-code for follow-up POC cleanup.

---

Authored by Claude with guidance and feedback from me. I haven't reviewed all of this myself yet. After the sample-plugin workshop at the conference, we'll address @@TODOs, fully review the code, and merge.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.87%. Comparing base (e7a17d1) to head (b3aa4d9).

Additional details and impacted files

@@ Coverage Diff @@## main #44 +/- ##
===========================================
- Coverage 93.04% 81.87% -11.17% 
===========================================
Files 16 16 Lines 503 673 +170 Branches 18 33 +15 ===========================================
+ Hits 468 551 +83 - Misses 24 111 +87 
Partials 11 11 

Flag	Coverage Δ
unittests	81.87% <ø> (-11.17%)	⬇️

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