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/App.tsx

@@ -10,6 +10,7 @@ import Users from './pages/Users';
 import Organizations from './pages/Organizations';
 import Settings from './pages/Settings';
 import Workflow from './pages/Workflow';
+import Messages from './pages/Messages';
 import { useAuthStore } from './stores/auth';
 import { useAppStore } from './stores/app';
 
@@ -48,6 +49,7 @@ export default function App() {
                     <Route path="/roles" element={<Roles />} />
                     <Route path="/organizations" element={<Organizations />} />
                     <Route path="/workflow" element={<Workflow />} />
+                    <Route path="/messages" element={<Messages />} />
                     <Route path="/settings" element={<Settings />} />
                   </Routes>
                 </MainLayout>