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/api/messageTemplates.ts

@@ -0,0 +1,40 @@
+import client from './client';
+import type { PaginatedResponse } from './users';
+
+export interface MessageTemplateInfo {
+  id: string;
+  tenant_id: string;
+  name: string;
+  code: string;
+  channel: string;
+  title_template: string;
+  body_template: string;
+  language: string;
+  created_at: string;
+  updated_at: string;
+}
+
+export interface CreateTemplateRequest {
+  name: string;
+  code: string;
+  channel?: string;
+  title_template: string;
+  body_template: string;
+  language?: string;
+}
+
+export async function listTemplates(page = 1, pageSize = 20) {
+  const { data } = await client.get<{ success: boolean; data: PaginatedResponse<MessageTemplateInfo> }>(
+    '/message-templates',
+    { params: { page, page_size: pageSize } },
+  );
+  return data.data;
+}
+
+export async function createTemplate(req: CreateTemplateRequest) {
+  const { data } = await client.post<{ success: boolean; data: MessageTemplateInfo }>(
+    '/message-templates',
+    req,
+  );
+  return data.data;
+}