feat: Settings changes now apply to real service config files and restart containers

Each service manager now has apply_config() that writes to the actual config:
- network: dhcp_range → dnsmasq.conf (reload cell-dhcp), ntp_servers → chrony.conf
  (restart cell-ntp), domain → dnsmasq.conf domain= line
- email: domain → mailserver.env OVERRIDE_HOSTNAME + POSTMASTER_ADDRESS,
  restart cell-mail
- wireguard: port/address/private_key → wg0.conf ListenPort/Address/PrivateKey,
  restart cell-wireguard
- calendar: port → radicale config hosts=, restart cell-radicale

PUT /api/config now calls apply_config() after persisting JSON, and returns
{restarted: [...], warnings: [...]} so Settings UI can show which containers
were restarted. _restart_container() helper added to BaseServiceManager.

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

api/email_manager.py

@@ -207,6 +207,51 @@ class EmailManager(BaseServiceManager):
         except Exception as e:
             logger.error(f"Error saving domain config: {e}")
 
+    def apply_config(self, config: Dict[str, Any]) -> Dict[str, Any]:
+        """Write config to mailserver.env and restart cell-mail."""
+        restarted = []
+        warnings = []
+        env_file = os.path.join(self.config_dir, 'mail', 'mailserver.env')
+        try:
+            # Read existing env file
+            env_lines = []
+            if os.path.exists(env_file):
+                with open(env_file) as f:
+                    env_lines = f.readlines()
+
+            def _set_env(lines, key, value):
+                found = False
+                result = []
+                for l in lines:
+                    if l.startswith(f'{key}='):
+                        result.append(f'{key}={value}\n')
+                        found = True
+                    else:
+                        result.append(l)
+                if not found:
+                    result.append(f'{key}={value}\n')
+                return result
+
+            changed = False
+            if 'domain' in config and config['domain']:
+                domain = config['domain']
+                env_lines = _set_env(env_lines, 'OVERRIDE_HOSTNAME', f'mail.{domain}')
+                env_lines = _set_env(env_lines, 'POSTMASTER_ADDRESS', f'admin@{domain}')
+                # Also persist to domain_config_file
+                self._save_domain_config({'domain': domain})
+                changed = True
+
+            if changed:
+                with open(env_file, 'w') as f:
+                    f.writelines(env_lines)
+                self._restart_container('cell-mail')
+                restarted.append('cell-mail')
+        except Exception as e:
+            warnings.append(f"mailserver.env update failed: {e}")
+            logger.error(f"apply_config error: {e}")
+
+        return {'restarted': restarted, 'warnings': warnings}
+
     def get_email_status(self) -> Dict[str, Any]:
         """Get detailed email service status including postfix/dovecot state."""
         try: