feat: make DDNS domain_name the effective domain across all services

- ConfigManager.get_effective_domain(): returns domain_name when DDNS
  active (pic_ngo/cloudflare/duckdns), domain otherwise. Used by all
  public-facing services so they use the real registered FQDN.
- ConfigManager.get_internal_domain(): always returns _identity.domain
  (CoreDNS zone name, dnsmasq, cell-link invites — stays internal).
- Silent migration: if domain_mode != lan and domain is generic "cell",
  auto-set to {cell_name}.local for unique CoreDNS zone naming.
- caddy_manager: fix custom_domain bug — cloudflare/http01 modes were
  reading identity.get('custom_domain') which never exists; now reads
  domain_name correctly.
- routes/config, app: expose effective_domain in GET /api/config and
  /api/status responses.
- email_manager, routes/email: use get_effective_domain() for
  OVERRIDE_HOSTNAME, POSTMASTER_ADDRESS, and new-user email defaults.
- ServiceBus.IDENTITY_CHANGED event: emitted from PUT /api/config and
  POST /api/ddns/register after identity writes; caddy_manager and
  email_manager subscribe to regenerate config automatically.
- Settings.jsx: hide Local Domain input in non-LAN modes; show
  read-only effective_domain with "managed by DDNS" badge and an
  Advanced toggle for the internal CoreDNS zone name.
- 11 new test classes covering all new helpers, event subscriptions,
  caddy/email handlers, and the custom_domain fix.

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

api/email_manager.py

@@ -19,7 +19,8 @@ logger = logging.getLogger(__name__)
 class EmailManager(BaseServiceManager):
     """Manages email service configuration and users"""
     
-    def __init__(self, data_dir: str = '/app/data', config_dir: str = '/app/config'):
+    def __init__(self, data_dir: str = '/app/data', config_dir: str = '/app/config',
+                 service_bus=None):
         super().__init__('email', data_dir, config_dir)
         self.email_data_dir = os.path.join(data_dir, 'email')
         self.email_dir = self.email_data_dir  # alias used by tests
@@ -33,6 +34,10 @@ class EmailManager(BaseServiceManager):
         self.safe_makedirs(self.dovecot_dir)
         self.safe_makedirs(os.path.dirname(self.domain_config_file))
 
+        if service_bus is not None:
+            from service_bus import EventType
+            service_bus.subscribe_to_event(EventType.IDENTITY_CHANGED, self._on_identity_changed)
+
     def _get_service_config(self) -> Dict[str, Any]:
         """Read configured ports/domain from service config file."""
         cfg = self.get_config()
@@ -252,6 +257,15 @@ class EmailManager(BaseServiceManager):
 
         return {'restarted': restarted, 'warnings': warnings}
 
+    def _on_identity_changed(self, event) -> None:
+        """Regenerate email config when cell identity changes."""
+        try:
+            effective = event.data.get('effective_domain')
+            if effective:
+                self.apply_config({'domain': effective})
+        except Exception as exc:
+            self.logger.warning('email_manager identity_changed handler failed: %s', exc)
+
     def get_email_status(self) -> Dict[str, Any]:
         """Get detailed email service status including postfix/dovecot state."""
         try: