Questions: add min-length to string/text type questions (#6488)

* Questions: add min-length to string/text type questions

* fix test

src/pretix/base/migrations/0310_question_valid_string_length_min.py

@@ -0,0 +1,19 @@
+# Generated by Django 5.2.12 on 2026-08-19 11:31
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("pretixbase", "0309_alter_questionanswer_unique_together_and_more"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="question",
+            name="valid_string_length_min",
+            field=models.PositiveIntegerField(null=True),
+        ),
+    ]