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>

crates/erp-message/src/message_state.rs

@@ -0,0 +1,9 @@
+use erp_core::events::EventBus;
+use sea_orm::DatabaseConnection;
+
+/// 消息中心模块状态，通过 FromRef 从 AppState 提取。
+#[derive(Clone)]
+pub struct MessageState {
+    pub db: DatabaseConnection,
+    pub event_bus: EventBus,
+}