[BACKEND] Fix CourseMembership.clean() crash on unsaved course & enforce teacher requirement

[smattymatty]

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Problem

Two related issues with course creation and membership validation:

1. CourseMembership.clean() crashes with unsaved Course instances

When creating a course through the Django admin with an inline membership (e.g., adding yourself as teacher at the same time), the clean() method on CourseMembership raises:

ValueError: Model instances passed to related filters must be saved.

Root cause: CourseMembership.clean() (line 216 in courses/models.py) runs CourseMembership.objects.filter(user=self.user, course=self.course) — but when the course is being created alongside the membership via admin inlines, self.course hasn't been saved yet and has no PK, so Django can't use it in a queryset filter.

2. No enforcement that a course must have at least one teacher

• The API correctly auto-creates a teacher membership for the course creator (views.py line 220-225), but this is only application-level logic — there's no model-level guarantee.
• The admin allows creating a course with zero members, leaving it in a broken state (no teacher).
• There's no validation preventing removal of the last teacher through the admin interface (the API has is_last_teacher() checks, but the admin bypasses those).

Requirements

1. Fix the crash: Guard CourseMembership.clean() so it skips the duplicate-membership DB query when either self.user or self.course is unsaved (no PK). The UniqueConstraint on ["user", "course"] already prevents duplicates at the DB level, so the clean() check is a user-friendly validation layer — it's fine to skip it when the instances aren't persisted yet.

2. Enforce teacher requirement in admin: Override CourseAdmin.save_related() (or use a similar hook) to ensure that after all inlines are saved, the course has at least one teacher membership. If not, raise a ValidationError preventing the save.

3. All existing tests must continue to pass. The API flow (auto-creating teacher membership on course creation) should remain unchanged.

Architecture Context

• courses/models.py — CourseMembership.clean() is the crash site. The Course.is_last_teacher() method already exists for last-teacher checks in the API views.
• courses/admin.py — CourseAdmin uses CourseMembershipInline as a TabularInline. The save_related() hook runs after the course and all inlines have been saved, making it the right place to validate teacher existence.
• courses/views.py — The API's post() method already handles auto-teacher-creation and should not be changed.

Files to be Altered

• backend/EduLite/courses/models.py — Guard CourseMembership.clean() against unsaved instances
• backend/EduLite/courses/admin.py — Add teacher-existence validation in CourseAdmin
• backend/EduLite/courses/tests/ — Add tests for both fixes

Testing Requirements

• Test 1: Creating a CourseMembership via admin inline on a new course does not crash
• Test 2: Creating a course via admin without a teacher membership shows a validation error
• Test 3: Removing the last teacher via admin is blocked
• Test 4: Existing API tests still pass (the auto-teacher flow is unchanged)
• Test 5: CourseMembership.clean() still catches duplicate memberships when both instances are saved

Additional Context (Optional)

• Discovered while manually testing the Course Detail Page frontend (Issue [FRONTEND] Build Course Detail Page with Membership and Module Management #249) — needed to create courses via admin to generate test data.
• The UniqueConstraint on ["user", "course"] acts as a DB-level safety net for the duplicate check, so skipping the clean() query for unsaved instances is safe.