Enforce uniqueness of order codes and ticket secrets (#3988)

* Enforce uniqueness of order codes and ticket secrets

* Fix test cases which created orders with identical codes

---------

Co-authored-by: Mira Weller <weller@rami.io>

src/pretix/base/migrations/0258_uniq_indx.py

@@ -0,0 +1,48 @@
+# Generated by Django 4.2.10 on 2024-03-15 09:59
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("pretixbase", "0257_item_default_price_not_null"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="order",
+            name="organizer",
+            field=models.ForeignKey(
+                null=True,
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name="orders",
+                to="pretixbase.organizer",
+            ),
+        ),
+        migrations.AddField(
+            model_name="orderposition",
+            name="organizer",
+            field=models.ForeignKey(
+                null=True,
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name="order_positions",
+                to="pretixbase.organizer",
+            ),
+        ),
+        migrations.AddConstraint(
+            model_name="order",
+            constraint=models.UniqueConstraint(
+                fields=("organizer", "code"), name="order_organizer_code_uniq"
+            ),
+        ),
+        migrations.AddConstraint(
+            model_name="orderposition",
+            constraint=models.UniqueConstraint(
+                models.F("organizer"),
+                models.F("secret"),
+                name="orderposition_organizer_secret_uniq",
+            ),
+        ),
+    ]