Add ClubMembership enhancements, CSV import command, and related tests

- Extend the ClubMembership model with `club`, `member`, and optional `license` fields, along with relevant constraints and ordering.
- Implement verbose names for Club and ClubMembership models and update admin configurations for better display and filtering.
- Add a `import_members_csv` management command for batch importing members, clubs, and memberships from a CSV file.
- Include extensive tests for the `import_members_csv` command, ClubMembership model, and Club model.
- Refactor related migrations, services, and test structure.

club/migrations/0004_alter_clubmembership_license.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.6 on 2026-07-05 20:54
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0003_alter_clubmembership_options_alter_club_name_and_more'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='clubmembership',
+            name='license',
+            field=models.CharField(blank=True, max_length=250, verbose_name='license'),
+        ),
+    ]