BE-CAL-API-002: Implement calendar day-view query API

- Add GET /calendar/day endpoint with optional ?date= query param
- Returns unified CalendarDayResponse merging real slots + virtual plan slots
- New CalendarSlotItem schema supports both real (id) and virtual (virtual_id) slots
- Excludes inactive slots (skipped/aborted) from results
- All slots sorted by scheduled_at ascending

plans/TASKLIST.md

@@ -144,10 +144,10 @@
   - [x] 支持 job/system/event_data
   - [x] 接入 overlap 与 workload warning
 
-- [ ] BE-CAL-API-002:实现某日 Calendar 查询 API(backend only)
-  - [ ] 返回真实 slot
-  - [ ] 合成当日 plan 虚拟 slot
-  - [ ] 输出统一排序结构
+- [x] BE-CAL-API-002:实现某日 Calendar 查询 API（backend only）
+  - [x] 返回真实 slot
+  - [x] 合成当日 plan 虚拟 slot
+  - [x] 输出统一排序结构
 
 - [ ] BE-CAL-API-003:实现 Calendar 编辑 API(backend only)
   - [ ] 支持真实 slot 编辑