feat(billing): add missing payment methods and configurations

- Added migration to include global payment method configurations for Stripe and PayPal (both disabled).
- Ensured existing payment methods like bank transfer and manual payment are correctly configured.
- Added database constraints and indexes for improved data integrity in billing models.
- Introduced foreign key relationship between CreditTransaction and Payment models.
- Added webhook configuration fields to PaymentMethodConfig for future payment gateway integrations.
- Updated SignUpFormUnified component to handle payment method selection based on user country and plan.
- Implemented PaymentHistory component to display user's payment history with status indicators.

backend/igny8_core/auth/migrations/0012_fix_subscription_constraints.py

@@ -0,0 +1,47 @@
+# Generated migration to fix subscription constraints
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('igny8_core_auth', '0011_remove_subscription_payment_method'),
+    ]
+
+    operations = [
+        # Add unique constraint on tenant_id at database level
+        migrations.RunSQL(
+            sql="""
+            CREATE UNIQUE INDEX IF NOT EXISTS igny8_subscriptions_tenant_id_unique 
+            ON igny8_subscriptions(tenant_id);
+            """,
+            reverse_sql="""
+            DROP INDEX IF EXISTS igny8_subscriptions_tenant_id_unique;
+            """
+        ),
+        
+        # Make plan field required (non-nullable)
+        # First set default plan (ID 1 - Free Plan) for any null values
+        migrations.RunSQL(
+            sql="""
+            UPDATE igny8_subscriptions 
+            SET plan_id = 1 
+            WHERE plan_id IS NULL;
+            """,
+            reverse_sql=migrations.RunSQL.noop
+        ),
+        
+        # Now alter the field to be non-nullable
+        migrations.AlterField(
+            model_name='subscription',
+            name='plan',
+            field=models.ForeignKey(
+                on_delete=django.db.models.deletion.PROTECT,
+                related_name='subscriptions',
+                to='igny8_core_auth.plan',
+                help_text='Subscription plan (tracks historical plan even if account changes plan)'
+            ),
+        ),
+    ]