feat: 集成 apisbo.com 中国节假日 API 自动获取+缓存

- 新增 holidays.py 服务：按年获取节假日数据，缓存到 system_config
- 区分 holiday（工作日假日）和 workday（调休上班日）两种类型
- is_working_day() 支持调休判断：周末在 workdays 集合中视为工作日
- 启动时自动从 API 刷新当年+明年数据，失败则使用缓存
- 系统设置新增「从 API 刷新」按钮，支局长可手动触发
- 支持 2010-2026 年数据，API 免费免鉴权

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/main.py

@@ -8,6 +8,7 @@ from app.api import router as api_router
 from app.api import auth, users, customers, visits, work_plans, mini_business, key_visits
 from app.api import dashboard, upload, export, import_data, wecom, daily_notes, ai_summary, system_config, leaves
 from app.models.system_config import SystemConfig
+from app.services.holidays import refresh_holidays
 from app.services.scheduler_manager import start_scheduler, shutdown_scheduler
 
 
@@ -84,6 +85,11 @@ async def lifespan(app: FastAPI):
                 notification_hour, notification_minute = int(parts[0]), int(parts[1])
             except (ValueError, IndexError):
                 pass
+        # Auto-refresh Chinese holiday data from API on startup
+        try:
+            await refresh_holidays(db)
+        except Exception:
+            pass  # Use cached data if API unavailable
 
     # Start daily reporting scheduler
     start_scheduler(notification_hour, notification_minute)