feat: add VisitedCity model and serializer, update admin interface, and enhance city visit tracking functionality

backend/server/worldtravel/migrations/0013_visitedcity.py

@@ -0,0 +1,24 @@
+# Generated by Django 5.0.8 on 2025-01-09 17:00
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('worldtravel', '0012_city'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='VisitedCity',
+            fields=[
+                ('id', models.AutoField(primary_key=True, serialize=False)),
+                ('city', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='worldtravel.city')),
+                ('user_id', models.ForeignKey(default=1, on_delete=django.db.models.deletion.CASCADE, to=settings.AUTH_USER_MODEL)),
+            ],
+        ),
+    ]