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/dto.rs

@@ -30,6 +30,7 @@ pub struct MessageResp {
     pub sent_at: Option<DateTime<Utc>>,
     pub created_at: DateTime<Utc>,
     pub updated_at: DateTime<Utc>,
+    pub version: i32,
 }
 
 /// 发送消息请求
@@ -162,6 +163,7 @@ pub struct MessageSubscriptionResp {
     pub dnd_end: Option<String>,
     pub created_at: DateTime<Utc>,
     pub updated_at: DateTime<Utc>,
+    pub version: i32,
 }
 
 /// 更新消息订阅偏好请求
@@ -172,4 +174,5 @@ pub struct UpdateSubscriptionReq {
     pub dnd_enabled: Option<bool>,
     pub dnd_start: Option<String>,
     pub dnd_end: Option<String>,
+    pub version: i32,
 }