feat(ai): 建议状态生命周期 — 转换验证 + 执行端点 + 事件发布

建议（ai_suggestion）原有状态枚举完整但缺乏生命周期管理：
- 无转换验证（可从 Rejected 跳到 Approved）
- 无执行端点（护士无法标记"已执行"）
- 无状态变更事件

变更：
1. SuggestionStatus.can_transition_to() — 仅允许合法单向转换
   Pending → Approved/Rejected/Expired → Approved → Executed/Rejected/Expired
2. SuggestionService.execute_suggestion() — 记录执行结果
3. SuggestionService.expire_stale_suggestions() — 批量过期超时建议
4. POST /ai/suggestions/{id}/execute — 新执行端点
5. publish_status_event() — 状态变更时发布 ai.suggestion.status_changed 事件
6. 9 个新单元测试覆盖所有转换规则

crates/erp-ai/src/dto/suggestion.rs

@@ -67,6 +67,19 @@ impl SuggestionStatus {
             Self::ParseFailed => "parse_failed",
         }
     }
+
+    /// 合法状态转换：仅允许相邻状态单向流转
+    pub fn can_transition_to(&self, target: SuggestionStatus) -> bool {
+        matches!(
+            (self, target),
+            (Self::Pending, Self::Approved)
+                | (Self::Pending, Self::Rejected)
+                | (Self::Pending, Self::Expired)
+                | (Self::Approved, Self::Executed)
+                | (Self::Approved, Self::Rejected)
+                | (Self::Approved, Self::Expired)
+        )
+    }
 }
 
 /// AI 输出的单条结构化建议