fix: add built-in service subdomains to DNS zone on startup

_build_dns_records() only hardcoded 'api' and 'webui', relying on the
optional service registry for the rest. Built-in services (calendar,
files, mail, webdav) were never registered, so they were absent from
the zone file and tests querying webdav.<domain> via CoreDNS got
NXDOMAIN.

Add _BUILTIN_SERVICE_SUBDOMAINS constant and include those names in
every zone build. Also update _stale and apply_cell_name exclusion
sets so DDNS mode correctly removes them from the parent zone.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_peer_dashboard_services.py

@@ -501,23 +501,15 @@ class TestDNSZoneRecords:
         )
 
     def test_service_records_absent_without_registry(self):
-        """Without a registry, service subdomain records are not generated.
-
-        Phase 2: service DNS records only exist when a service is installed
-        and the registry reports it. The hardcoded fallback is gone.
-        """
+        """Built-in services always get DNS records; optional services require a registry."""
         records = self._records()
         names = {r['name'] for r in records}
-        assert 'calendar' not in names, \
-            'calendar DNS record must not appear without a registry'
-        assert 'files' not in names, \
-            'files DNS record must not appear without a registry'
-        assert 'mail' not in names, \
-            'mail DNS record must not appear without a registry'
+        # Built-in services are always present
+        for svc in ('calendar', 'files', 'mail', 'webdav'):
+            assert svc in names, f'{svc} DNS record must always be generated'
+        # Non-built-in names are only generated when a registry is wired
         assert 'webmail' not in names, \
             'webmail DNS record must not appear without a registry'
-        assert 'webdav' not in names, \
-            'webdav DNS record must not appear without a registry'
 
     def test_service_records_present_with_registry(self):
         """With a registry that provides calendar/mail/files, all resolve to WG IP."""