feat(工作日): 新增工作小时计算工具 + 节假日表与管理接口

- time_utils 新增 to_beijing(统一naive按UTC转北京时间) + working_duration_hours(排除周末/节假日)
- Holiday 模型 + alembic 迁移建 holidays 表
- GET/POST/DELETE /holidays 节假日管理接口，可随时配置放假日期

backend/app/api/v1/router.py

@@ -13,6 +13,7 @@ from app.api.v1.endpoints.records import router as records_router
 from app.api.v1.endpoints.notifications import router as notifications_router
 from app.api.v1.endpoints.app_version import router as app_version_router
 from app.api.v1.endpoints.analytics import router as analytics_router
+from app.api.v1.endpoints.holidays import router as holidays_router
 
 api_router = APIRouter()
 
@@ -29,3 +30,4 @@ api_router.include_router(records_router)
 api_router.include_router(notifications_router)
 api_router.include_router(app_version_router)
 api_router.include_router(analytics_router)
+api_router.include_router(holidays_router)