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/models.py

@@ -7,7 +7,7 @@ import netbox_dns.models
 class IntegerKeyValueSetting(models.Model):
     _netbox_private = True
 
-    key = models.CharField(max_length=64)
+    key = models.CharField(max_length=64, unique=True)
     value = models.IntegerField()
 
     def __str__(self):