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/utils/timezone.py

@@ -13,19 +13,30 @@ def parse_date(s: str) -> date:
     return date.fromisoformat(s[:10])
 
 
-def is_working_day(d: date | None = None, holidays: set[date] | None = None) -> bool:
+def is_working_day(d: date | None = None, holidays: set[date] | None = None, workdays: set[date] | None = None) -> bool:
     """Check if a date is a Chinese working day.
 
     Returns False for:
-    - Saturdays and Sundays (weekday >= 5)
-    - Dates in the holidays set (configurable via system_config)
+    - Saturdays and Sundays (weekday >= 5), UNLESS in the workdays set (调休)
+    - Dates in the holidays set (weekday holidays like 春节/国庆)
 
-    Note: makeup workdays (调休, where Saturday/Sunday becomes a workday)
-    are not currently handled. If needed, add a 'workdays' config set.
+    Returns True for:
+    - Monday-Friday, unless in holidays set
+    - Saturday/Sunday that are in the workdays set (调休 makeup days)
     """
     d = d or today_cst()
-    if d.weekday() >= 5:  # Saturday or Sunday
+    is_weekend = d.weekday() >= 5
+
+    # 调休: weekend that becomes a workday
+    if is_weekend and workdays and d in workdays:
+        return True
+
+    # Normal weekend
+    if is_weekend:
         return False
+
+    # Weekday holiday
     if holidays and d in holidays:
         return False
+
     return True