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/managers.py

@@ -45,7 +45,7 @@ log_manager  = LogManager(log_dir='./data/logs')
 network_manager    = NetworkManager(data_dir=DATA_DIR, config_dir=CONFIG_DIR)
 wireguard_manager  = WireGuardManager(data_dir=DATA_DIR, config_dir=CONFIG_DIR)
 peer_registry      = PeerRegistry(data_dir=DATA_DIR, config_dir=CONFIG_DIR)
-email_manager      = EmailManager(data_dir=DATA_DIR, config_dir=CONFIG_DIR)
+email_manager      = EmailManager(data_dir=DATA_DIR, config_dir=CONFIG_DIR, service_bus=service_bus)
 calendar_manager   = CalendarManager(data_dir=DATA_DIR, config_dir=CONFIG_DIR)
 file_manager       = FileManager(data_dir=DATA_DIR, config_dir=CONFIG_DIR)
 routing_manager    = RoutingManager(data_dir=DATA_DIR, config_dir=CONFIG_DIR)
@@ -58,7 +58,7 @@ cell_link_manager  = CellLinkManager(
 )
 auth_manager = AuthManager(data_dir=DATA_DIR, config_dir=CONFIG_DIR)
 setup_manager = SetupManager(config_manager=config_manager, auth_manager=auth_manager)
-caddy_manager = CaddyManager(config_manager=config_manager, data_dir=DATA_DIR, config_dir=CONFIG_DIR)
+caddy_manager = CaddyManager(config_manager=config_manager, data_dir=DATA_DIR, config_dir=CONFIG_DIR, service_bus=service_bus)
 ddns_manager = DDNSManager(config_manager=config_manager, data_dir=DATA_DIR, config_dir=CONFIG_DIR)
 connectivity_manager = ConnectivityManager(
     config_manager=config_manager,