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/module.rs

@@ -0,0 +1,99 @@
+use axum::Router;
+use axum::routing::{delete, get, put};
+use uuid::Uuid;
+
+use erp_core::error::AppResult;
+use erp_core::events::EventBus;
+use erp_core::module::ErpModule;
+
+use crate::handler::{
+    message_handler, subscription_handler, template_handler,
+};
+
+/// 消息中心模块，实现 ErpModule trait。
+pub struct MessageModule;
+
+impl MessageModule {
+    pub fn new() -> Self {
+        Self
+    }
+
+    /// 构建需要认证的路由。
+    pub fn protected_routes<S>() -> Router<S>
+    where
+        crate::message_state::MessageState: axum::extract::FromRef<S>,
+        S: Clone + Send + Sync + 'static,
+    {
+        Router::new()
+            // 消息路由
+            .route(
+                "/messages",
+                get(message_handler::list_messages).post(message_handler::send_message),
+            )
+            .route(
+                "/messages/unread-count",
+                get(message_handler::unread_count),
+            )
+            .route(
+                "/messages/{id}/read",
+                put(message_handler::mark_read),
+            )
+            .route(
+                "/messages/read-all",
+                put(message_handler::mark_all_read),
+            )
+            .route(
+                "/messages/{id}",
+                delete(message_handler::delete_message),
+            )
+            // 模板路由
+            .route(
+                "/message-templates",
+                get(template_handler::list_templates).post(template_handler::create_template),
+            )
+            // 订阅偏好路由
+            .route(
+                "/message-subscriptions",
+                put(subscription_handler::update_subscription),
+            )
+    }
+}
+
+impl Default for MessageModule {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[async_trait::async_trait]
+impl ErpModule for MessageModule {
+    fn name(&self) -> &str {
+        "message"
+    }
+
+    fn version(&self) -> &str {
+        env!("CARGO_PKG_VERSION")
+    }
+
+    fn dependencies(&self) -> Vec<&str> {
+        vec!["auth"]
+    }
+
+    fn register_routes(&self, router: Router) -> Router {
+        router
+    }
+
+    fn register_event_handlers(&self, _bus: &EventBus) {}
+
+    async fn on_tenant_created(&self, _tenant_id: Uuid) -> AppResult<()> {
+        Ok(())
+    }
+
+    async fn on_tenant_deleted(&self, _tenant_id: Uuid) -> AppResult<()> {
+        Ok(())
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}