BE-CAL-API-005: implement plan-schedule / plan-list API

- Add SchedulePlanCreate, SchedulePlanResponse, SchedulePlanListResponse schemas
- Add DayOfWeekEnum, MonthOfYearEnum schema enums
- Add POST /calendar/plans endpoint (create plan with hierarchy validation)
- Add GET /calendar/plans endpoint (list plans, optional include_inactive)
- Add GET /calendar/plans/{plan_id} endpoint (get single plan)
- Mark BE-CAL-API-005 complete in TASKLIST.md

plans/TASKLIST.md

@@ -159,10 +159,10 @@
   - [x] 支持虚拟 slot cancel 并触发物化
   - [x] 处理状态字段更新
 
-- [ ] BE-CAL-API-005:实现 plan-schedule / plan-list API(backend only)
-  - [ ] 创建计划
-  - [ ] 列出计划
-  - [ ] 返回计划规则字段
+- [x] BE-CAL-API-005:实现 plan-schedule / plan-list API(backend only)
+  - [x] 创建计划
+  - [x] 列出计划
+  - [x] 返回计划规则字段
 
 - [ ] BE-CAL-API-006:实现 plan-edit / plan-cancel API(backend only)
   - [ ] 编辑计划