feat(course): require thumbnail in CourseForm and enforce validation rules

- Made thumbnail field required in CourseForm.
- Added validation to prevent clearing existing thumbnail.
- Enforced requirement for uploading a thumbnail on creation or when no existing thumbnail is present.

apps/course/admin/course.py

@@ -112,6 +112,25 @@ class CourseForm(forms.ModelForm):
         super().__init__(*args, **kwargs)
         # Make short_description required
         self.fields['short_description'].required = True
+        # Make thumbnail required (show required star in add/change forms)
+        if 'thumbnail' in self.fields:
+            self.fields['thumbnail'].required = True
+
+    def clean(self):
+        cleaned_data = super().clean()
+        thumbnail = cleaned_data.get('thumbnail')
+        has_existing_thumbnail = bool(getattr(self.instance, 'thumbnail', None))
+
+        # Disallow clearing the existing thumbnail (must always have a value)
+        if thumbnail is False:
+            self.add_error('thumbnail', _('This field is required and cannot be cleared.'))
+            return cleaned_data
+
+        # On create or when no existing thumbnail, require uploading one
+        if (thumbnail is None or thumbnail == '') and not has_existing_thumbnail:
+            self.add_error('thumbnail', _('This field is required.'))
+
+        return cleaned_data
 
 
 class CourseAttachmentInline(StackedInline):