feat(course): update thumbnail field to be required

- Altered the thumbnail field in the Course model to remove null and blank options, making it mandatory.
- Added a migration to reflect this change in the database schema.

apps/course/migrations/0007_alter_course_thumbnail.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.2.4 on 2025-09-16 14:50
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('course', '0006_participant_is_active'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='course',
+            name='thumbnail',
+            field=models.ImageField(default='1', upload_to='courses/thumbnails/', verbose_name='Thumbnail'),
+            preserve_default=False,
+        ),
+    ]

apps/course/models/course.py

@@ -69,7 +69,7 @@ class Course(models.Model):
         related_name="courses"
     )
     
-    thumbnail = models.ImageField(upload_to="courses/thumbnails/", null=True, blank=True, verbose_name=_('Thumbnail'))
+    thumbnail = models.ImageField(upload_to="courses/thumbnails/", verbose_name=_('Thumbnail'))
     video_type = models.CharField(
         max_length=20, 
         choices=VedioTypeChoices.choices,