Connect giftcards with customer accounts (#5126)

Connect giftcards with customer accounts, show giftcards during checkout and in account , show giftcard list in backend customer view

src/pretix/base/migrations/0292_giftcard_customer.py

@@ -0,0 +1,19 @@
+# Generated by Django 4.2.19 on 2025-05-19 11:06
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0291_alter_logentry_object_id'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='giftcard',
+            name='customer',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.PROTECT, related_name='customer_gift_cards', to='pretixbase.customer'),
+        ),
+    ]