BE-CAL-API-006: implement plan-edit and plan-cancel API endpoints

- PATCH /calendar/plans/{plan_id}: edit a recurring schedule plan
  - Validates period-parameter hierarchy after merge
  - Rejects edits to inactive (cancelled) plans
  - Detaches future materialized slots so they keep old data
  - Past materialized slots remain untouched

- POST /calendar/plans/{plan_id}/cancel: cancel (soft-delete) a plan
  - Sets is_active=False
  - Detaches future materialized slots (plan_id -> NULL)
  - Preserves past materialized slots, returns their IDs

- Added SchedulePlanEdit and SchedulePlanCancelResponse schemas
- Imported guard_plan_edit/cancel_no_past_retroaction from slot_immutability

plans/TASKLIST.md

@@ -164,10 +164,10 @@
   - [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
-  - [ ] 取消计划
-  - [ ] 不追溯已物化过去数据
+- [x] BE-CAL-API-006:实现 plan-edit / plan-cancel API（backend only）
+  - [x] 编辑计划
+  - [x] 取消计划
+  - [x] 不追溯已物化过去数据
 
 - [ ] BE-CAL-API-007:实现 date-list API(backend only)
   - [ ] 仅列出有已物化未来 slot 的日期