feat(message): add message center module (Phase 5)

Implement the complete message center with:
- Database migrations for message_templates, messages, message_subscriptions tables
- erp-message crate with entities, DTOs, services, handlers
- Message CRUD, send, read/unread tracking, soft delete
- Template management with variable interpolation
- Subscription preferences with DND support
- Frontend: messages page, notification panel, unread count badge
- Server integration with module registration and routing

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

apps/web/src/pages/Messages.tsx

@@ -0,0 +1,40 @@
+import { useState } from 'react';
+import { Tabs } from 'antd';
+import NotificationList from './messages/NotificationList';
+import MessageTemplates from './messages/MessageTemplates';
+import NotificationPreferences from './messages/NotificationPreferences';
+
+export default function Messages() {
+  const [activeKey, setActiveKey] = useState('all');
+
+  return (
+    <div>
+      <Tabs
+        activeKey={activeKey}
+        onChange={setActiveKey}
+        items={[
+          {
+            key: 'all',
+            label: '全部消息',
+            children: <NotificationList />,
+          },
+          {
+            key: 'unread',
+            label: '未读消息',
+            children: <NotificationList queryFilter={{ is_read: false }} />,
+          },
+          {
+            key: 'templates',
+            label: '消息模板',
+            children: <MessageTemplates />,
+          },
+          {
+            key: 'preferences',
+            label: '通知设置',
+            children: <NotificationPreferences />,
+          },
+        ]}
+      />
+    </div>
+  );
+}