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/handler/subscription_handler.rs

@@ -0,0 +1,31 @@
+use axum::extract::{Extension, State};
+use axum::extract::FromRef;
+use axum::Json;
+
+use erp_core::error::AppError;
+use erp_core::types::{ApiResponse, TenantContext};
+
+use crate::dto::UpdateSubscriptionReq;
+use crate::message_state::MessageState;
+use crate::service::subscription_service::SubscriptionService;
+
+/// 更新消息订阅偏好。
+pub async fn update_subscription<S>(
+    State(_state): State<MessageState>,
+    Extension(ctx): Extension<TenantContext>,
+    Json(req): Json<UpdateSubscriptionReq>,
+) -> Result<Json<ApiResponse<crate::dto::MessageSubscriptionResp>>, AppError>
+where
+    MessageState: FromRef<S>,
+    S: Clone + Send + Sync + 'static,
+{
+    let resp = SubscriptionService::upsert(
+        ctx.tenant_id,
+        ctx.user_id,
+        &req,
+        &_state.db,
+    )
+    .await?;
+
+    Ok(Json(ApiResponse::ok(resp)))
+}