Add unique constraint to IntegerKeyValueSetting.key

IntegerKeyValueSetting is used as a singleton-style key/value store:
callers do get(key=...) and get_or_create(key=..., ...) and assume at
most one row per key. The model, however, did not enforce uniqueness on
the key field, so nothing prevented duplicate rows at the database
level.

This was a real race for _create_soa_rdataset in catalog_zone_manager,
which calls get_or_create without select_for_update (unlike
increment_soa_serial, which does take a lock). Two concurrent AXFR
queries for a catalog zone could both pass the get_or_create existence
check and both insert a row for key='catalog-zone-soa-serial'. Every
subsequent IntegerKeyValueSetting.objects.get(key=...) call would then
raise MultipleObjectsReturned, breaking catalog-zone SOA rendering and
serial increments.

Add unique=True to the key CharField and ship migration 0003 to enforce
the invariant at the database level. Existing deployments are assumed to
have clean state (no duplicate keys); if duplicates exist, the migration
should be preceded by manual deduplication.

netbox_dns_bridge/migrations/0003_integerkeyvaluesetting_key_unique.py

@@ -0,0 +1,16 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("netbox_dns_bridge", "0002_alter_integerkeyvaluesetting_options_and_more"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="integerkeyvaluesetting",
+            name="key",
+            field=models.CharField(max_length=64, unique=True),
+        ),
+    ]