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/messages.ts

@@ -0,0 +1,88 @@
+import client from './client';
+import type { PaginatedResponse } from './users';
+
+export interface MessageInfo {
+  id: string;
+  tenant_id: string;
+  template_id?: string;
+  sender_id?: string;
+  sender_type: string;
+  recipient_id: string;
+  recipient_type: string;
+  title: string;
+  body: string;
+  priority: string;
+  business_type?: string;
+  business_id?: string;
+  is_read: boolean;
+  read_at?: string;
+  is_archived: boolean;
+  status: string;
+  sent_at?: string;
+  created_at: string;
+  updated_at: string;
+}
+
+export interface SendMessageRequest {
+  title: string;
+  body: string;
+  recipient_id: string;
+  recipient_type?: string;
+  priority?: string;
+  template_id?: string;
+  business_type?: string;
+  business_id?: string;
+}
+
+export interface MessageQuery {
+  page?: number;
+  page_size?: number;
+  is_read?: boolean;
+  priority?: string;
+  business_type?: string;
+  status?: string;
+}
+
+export async function listMessages(query: MessageQuery = {}) {
+  const { data } = await client.get<{ success: boolean; data: PaginatedResponse<MessageInfo> }>(
+    '/messages',
+    { params: { page: query.page ?? 1, page_size: query.page_size ?? 20, ...query } },
+  );
+  return data.data;
+}
+
+export async function getUnreadCount() {
+  const { data } = await client.get<{ success: boolean; data: { count: number } }>(
+    '/messages/unread-count',
+  );
+  return data.data;
+}
+
+export async function markRead(id: string) {
+  const { data } = await client.put<{ success: boolean }>(
+    `/messages/${id}/read`,
+  );
+  return data;
+}
+
+export async function markAllRead() {
+  const { data } = await client.put<{ success: boolean }>(
+    '/messages/read-all',
+  );
+  return data;
+}
+
+export async function deleteMessage(id: string) {
+  const { data } = await client.delete<{ success: boolean }>(
+    `/messages/${id}`,
+  );
+  return data;
+}
+
+export async function sendMessage(req: SendMessageRequest) {
+  const { data } = await client.post<{ success: boolean; data: MessageInfo }>(
+    '/messages/send',
+    req,
+  );
+  return data.data;
+}