feat: add course rating and review system API

[Anuoluwapo25]

Learners cannot rate or review courses.\n\nAction:\n- [ ] Add reviews table (address, course_id, rating 1-5, text, created_at)\n- [ ] Add migration\n- [ ] Add POST /api/courses/:id/reviews (must have completed course)\n- [ ] Add GET /api/courses/:id/reviews (public)\n- [ ] Include average rating in course list response\n- [ ] Prevent duplicate reviews from same address

[jerrymusaga]

@jerrymusaga has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello! Please I will like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @jerrymusaga to this issue.

[drips-wave]

Congratulations, @jerrymusaga! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @jerrymusaga: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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