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>

tests/test_config_manager.py

@@ -260,6 +260,92 @@ class TestConfigManager(unittest.TestCase):
             "import must not inject zero-filled entries for absent services")
 
 
+class TestGetEffectiveDomain(unittest.TestCase):
+    """Tests for ConfigManager.get_effective_domain and get_internal_domain."""
+
+    def setUp(self):
+        self.temp_dir = tempfile.mkdtemp()
+        self.config_file = os.path.join(self.temp_dir, 'cell_config.json')
+        self.data_dir = os.path.join(self.temp_dir, 'data')
+        os.makedirs(self.data_dir, exist_ok=True)
+
+    def tearDown(self):
+        shutil.rmtree(self.temp_dir)
+
+    def _make_cm(self, identity):
+        cm = ConfigManager(self.config_file, self.data_dir)
+        cm.configs['_identity'] = identity
+        return cm
+
+    def test_get_effective_domain_lan_mode(self):
+        cm = self._make_cm({'domain': 'home.local', 'domain_mode': 'lan'})
+        self.assertEqual(cm.get_effective_domain(), 'home.local')
+
+    def test_get_effective_domain_pic_ngo_uses_domain_name(self):
+        cm = self._make_cm({
+            'domain': 'home.local',
+            'domain_mode': 'pic_ngo',
+            'domain_name': 'home.pic.ngo',
+        })
+        self.assertEqual(cm.get_effective_domain(), 'home.pic.ngo')
+
+    def test_get_effective_domain_pic_ngo_fallback(self):
+        cm = self._make_cm({'domain': 'home.local', 'domain_mode': 'pic_ngo'})
+        self.assertEqual(cm.get_effective_domain(), 'home.local')
+
+    def test_get_internal_domain_always_returns_domain(self):
+        cm = self._make_cm({
+            'domain': 'home.local',
+            'domain_mode': 'pic_ngo',
+            'domain_name': 'home.pic.ngo',
+        })
+        self.assertEqual(cm.get_internal_domain(), 'home.local')
+
+    def test_get_internal_domain_ignores_domain_name(self):
+        cm = self._make_cm({
+            'domain': 'myzone.local',
+            'domain_mode': 'cloudflare',
+            'domain_name': 'example.com',
+        })
+        self.assertEqual(cm.get_internal_domain(), 'myzone.local')
+
+    def test_get_effective_domain_cloudflare_uses_domain_name(self):
+        cm = self._make_cm({
+            'domain': 'home.local',
+            'domain_mode': 'cloudflare',
+            'domain_name': 'example.com',
+        })
+        self.assertEqual(cm.get_effective_domain(), 'example.com')
+
+    def test_silent_migration_sets_unique_internal_domain(self):
+        """When DDNS is active and domain is the generic 'cell', migration sets cell_name.local."""
+        config_file2 = os.path.join(self.temp_dir, 'cell_config2.json')
+        with open(config_file2, 'w') as f:
+            json.dump({
+                '_identity': {
+                    'cell_name': 'alpha',
+                    'domain': 'cell',
+                    'domain_mode': 'pic_ngo',
+                }
+            }, f)
+        cm = ConfigManager(config_file2, self.data_dir)
+        self.assertEqual(cm.get_internal_domain(), 'alpha.local')
+
+    def test_silent_migration_does_not_touch_lan_mode(self):
+        """Migration must leave domain unchanged when domain_mode is 'lan'."""
+        config_file2 = os.path.join(self.temp_dir, 'cell_config3.json')
+        with open(config_file2, 'w') as f:
+            json.dump({
+                '_identity': {
+                    'cell_name': 'beta',
+                    'domain': 'cell',
+                    'domain_mode': 'lan',
+                }
+            }, f)
+        cm = ConfigManager(config_file2, self.data_dir)
+        self.assertEqual(cm.get_internal_domain(), 'cell')
+
+
 class TestNetworkManagerApply(unittest.TestCase):
     """Test apply_config / apply_domain actually write real config files."""