Fix get_or_create tuple unpacking in catalog_zone_manager

v1.5.6 fixed the wrong RelatedObjectDoesNotExist import and the catz
member-identifier creation, but two get_or_create() calls still used the
returned value directly instead of unpacking the (obj, created) tuple:

- increment_soa_serial(): `soa_serial_obj.value` on a tuple
- _create_soa_rdataset(): `.get_or_create(...).value` on a tuple

Both raised `'tuple' object has no attribute 'value'` at runtime — the
first on every zone save, the second on every catalog-zone (catz) query.
Unpack the tuple in both places.

netbox_dns_bridge/catalog_zone_manager.py

@@ -15,7 +15,7 @@ LOGGER = get_logger(__name__)
 def increment_soa_serial() -> int:
     with transaction.atomic():
         try:
-            soa_serial_obj = (
+            soa_serial_obj, _ = (
                 IntegerKeyValueSetting.objects.select_for_update().get_or_create(
                     key="catalog-zone-soa-serial",
                     defaults={"value": 1},
@@ -137,9 +137,10 @@ def update_member_identifier(zone: NBZone) -> None:
 
 def _create_soa_rdataset() -> dns.rdataset:
     try:
-        serial = IntegerKeyValueSetting.objects.get_or_create(
+        serial_obj, _ = IntegerKeyValueSetting.objects.get_or_create(
             key="catalog-zone-soa-serial", defaults={"value": 1}
-        ).value
+        )
+        serial = serial_obj.value
 
         # SOA Record components
         ttl = 0