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/caddy_manager.py

@@ -51,7 +51,8 @@ class CaddyManager(BaseServiceManager):
 
     def __init__(self, config_manager=None,
                  data_dir: str = '/app/data',
-                 config_dir: str = '/app/config'):
+                 config_dir: str = '/app/config',
+                 service_bus=None):
         super().__init__('caddy', data_dir, config_dir)
         self.config_manager = config_manager
         self.container_name = 'cell-caddy'
@@ -60,6 +61,10 @@ class CaddyManager(BaseServiceManager):
         # the caller restarts the container).
         self._health_failures = 0
 
+        if service_bus is not None:
+            from service_bus import EventType
+            service_bus.subscribe_to_event(EventType.IDENTITY_CHANGED, self._on_identity_changed)
+
     # ── BaseServiceManager required ───────────────────────────────────────
 
     def get_status(self) -> Dict[str, Any]:
@@ -126,14 +131,14 @@ class CaddyManager(BaseServiceManager):
         if domain_mode == 'pic_ngo':
             return self._caddyfile_pic_ngo(cell_name, service_routes, core_routes)
         if domain_mode == 'cloudflare':
-            custom_domain = identity.get('custom_domain', f'{cell_name}.local')
+            custom_domain = identity.get('domain_name', identity.get('domain', f'{cell_name}.local'))
             return self._caddyfile_cloudflare(
                 custom_domain, service_routes, core_routes
             )
         if domain_mode == 'duckdns':
             return self._caddyfile_duckdns(cell_name, service_routes, core_routes)
         if domain_mode == 'http01':
-            host = identity.get('custom_domain', f'{cell_name}.noip.me')
+            host = identity.get('domain_name', identity.get('domain', f'{cell_name}.noip.me'))
             return self._caddyfile_http01(host, installed_services, core_routes)
 
         # Fallback to lan so we always emit a valid Caddyfile.
@@ -379,6 +384,13 @@ class CaddyManager(BaseServiceManager):
         content = self.generate_caddyfile(identity, installed_services)
         return self.write_caddyfile(content)
 
+    def _on_identity_changed(self, event) -> None:
+        """Regenerate and reload the Caddyfile when cell identity changes."""
+        try:
+            self.regenerate_with_installed([])
+        except Exception as exc:
+            self.logger.warning('caddy_manager identity_changed handler failed: %s', exc)
+
     def get_cert_status(self) -> Dict[str, Any]:
         """Return TLS cert status from identity['tls'] if present."""
         default = {'status': 'unknown', 'expiry': None, 'days_remaining': None}