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/error.rs

@@ -0,0 +1,41 @@
+use erp_core::error::AppError;
+
+/// 消息中心模块错误类型。
+#[derive(Debug, thiserror::Error)]
+pub enum MessageError {
+    #[error("验证失败: {0}")]
+    Validation(String),
+
+    #[error("未找到: {0}")]
+    NotFound(String),
+
+    #[error("模板编码已存在: {0}")]
+    DuplicateTemplateCode(String),
+
+    #[error("渲染失败: {0}")]
+    TemplateRenderError(String),
+}
+
+impl From<MessageError> for AppError {
+    fn from(err: MessageError) -> Self {
+        match err {
+            MessageError::Validation(msg) => AppError::Validation(msg),
+            MessageError::NotFound(msg) => AppError::NotFound(msg),
+            MessageError::DuplicateTemplateCode(msg) => AppError::Conflict(msg),
+            MessageError::TemplateRenderError(msg) => AppError::Internal(msg),
+        }
+    }
+}
+
+impl From<sea_orm::TransactionError<MessageError>> for MessageError {
+    fn from(err: sea_orm::TransactionError<MessageError>) -> Self {
+        match err {
+            sea_orm::TransactionError::Connection(db_err) => {
+                MessageError::Validation(db_err.to_string())
+            }
+            sea_orm::TransactionError::Transaction(msg_err) => msg_err,
+        }
+    }
+}
+
+pub type MessageResult<T> = Result<T, MessageError>;