feat(core): implement optimistic locking across all entities

Add VersionMismatch error variant and check_version() helper to erp-core.
All 13 mutable entities now enforce version checking on update/delete:
- erp-auth: user, role, organization, department, position
- erp-config: dictionary, dictionary_item, menu, setting, numbering_rule
- erp-workflow: process_definition, process_instance, task
- erp-message: message, message_subscription

Update DTOs to expose version in responses and require version in update
requests. HTTP 409 Conflict returned on version mismatch.

crates/erp-message/src/error.rs

@@ -14,6 +14,9 @@ pub enum MessageError {
 
     #[error("渲染失败: {0}")]
     TemplateRenderError(String),
+
+    #[error("版本冲突: 数据已被其他操作修改，请刷新后重试")]
+    VersionMismatch,
 }
 
 impl From<MessageError> for AppError {
@@ -23,6 +26,7 @@ impl From<MessageError> for AppError {
             MessageError::NotFound(msg) => AppError::NotFound(msg),
             MessageError::DuplicateTemplateCode(msg) => AppError::Conflict(msg),
             MessageError::TemplateRenderError(msg) => AppError::Internal(msg),
+            MessageError::VersionMismatch => AppError::VersionMismatch,
         }
     }
 }