fix: 统一所有累计时长计算口径（必须 JOIN 到有效 activities）

- 统一口径定义：所有累计/总/人均/合计时长度量，一律只
  计算 hours 行中 activity_id 存在于 activities 表中的记录
  （排除 FK pragma 关闭时写入导致的幽灵 hours 行）

- server/users.ts：
  · GET /（用户列表列的累计时长）
    原 SELECT SUM(h.hours) FROM hours h → 增加 EXISTS 子查询
    『AND EXISTS (SELECT 1 FROM activities a WHERE a.id=h.activity_id)』
  · GET /:id 详情卡 total_hours_raw（给 enrichUser 用的原始数）
    同样加 EXISTS 校验，然后仍用详情 activities 列表 reduce 覆
    盖，保证卡片累计 = 列表合计 100% 一致，且与列表 SQL 同口径

- server/export.ts：
  · export include_user_totals（导出报告的部员累计时长排行）
    原 LEFT JOIN hours 直接 SUM → 改为 LEFT JOIN 预聚合子查
    询 vh，vh 内先按 EXISTS(activities) 过滤，再 SUM/COUNT
  · export include_activity_matrix（矩阵 _total、预览 KPI、
    导出 Excel 的总时长列）
    hoursRows 全表 SELECT → 增加 WHERE EXISTS(activities) 过
    滤，matrix/remarkMatrix 只聚合有效活动行，再派生的用户级
    _total 自然与 user_totals 同口径

- client/Export.tsx overviewCalc（导出报告顶部的累计总时长/
  人均时长 KPI 卡片，之前只依赖 preview.hours.reduce）：
  · 优先级：
    ① 有 user_totals → 用后端同口径返回的 u.total_hours 聚合
      （最靠谱）
    ② 否则有 activity_matrix → 用 matrix.user._total 聚合
      （两者都基于同一 valid_hours 过滤）
    ③ Fallback：preview.hours.reduce（仅在只勾了明细单且没
      勾排行/矩阵时才用）
  · 计算完 totalHours 再 toFixed(1) 去浮点数误差，avgHours
    同步 toFixed(1)

- client/user/Dashboard.tsx（部员侧义工号查询的个人累计页）：
  · 累计义工时长卡下半部新增『明细表合计』实时校验行：
    data.activities.reduce 求和直接显示在卡片上，与大号累计
    值左右对照，一眼确认有没有不一致

server/src/routes/export.ts

@@ -66,11 +66,17 @@ router.post('/', authMiddleware, (req: AuthRequest, res) => {
                 FROM user_groups ug2
                 LEFT JOIN work_groups g2 ON ug2.group_id = g2.id
                 WHERE ug2.user_id = u.id) AS _group_names_csv,
-             COALESCE(SUM(h.hours), 0) AS total_hours,
-             COUNT(DISTINCT h.activity_id) AS activity_count
+             COALESCE(vh.total_hours, 0) AS total_hours,
+             COALESCE(vh.activity_count, 0) AS activity_count
       FROM users u
-      LEFT JOIN hours h ON h.user_id = u.id
-      GROUP BY u.id
+      LEFT JOIN (
+        SELECT user_id,
+               SUM(hours) AS total_hours,
+               COUNT(DISTINCT activity_id) AS activity_count
+          FROM hours h
+         WHERE EXISTS (SELECT 1 FROM activities a WHERE a.id = h.activity_id)
+         GROUP BY user_id
+      ) vh ON vh.user_id = u.id
       ORDER BY total_hours DESC, u.class, u.name
     `).all();
     result.user_totals = withGroupNames(rows).map(({ _group_names_csv: _, ...rest }) => rest);
@@ -112,7 +118,8 @@ router.post('/', authMiddleware, (req: AuthRequest, res) => {
     `).all() as any[];
     const users = withGroupNames(userRows);
     const hoursRows = db.prepareCompat(`
-      SELECT user_id, activity_id, hours, remark FROM hours
+      SELECT user_id, activity_id, hours, remark FROM hours h
+       WHERE EXISTS (SELECT 1 FROM activities a WHERE a.id = h.activity_id)
     `).all() as any[];
     const matrix: Record<string, number> = {};
     const remarkMatrix: Record<string, string> = {};

server/src/routes/users.ts

@@ -31,7 +31,11 @@ router.get('/', authMiddleware, (req: AuthRequest, res: Response) => {
     SELECT u.id, u.class, u.name, u.volunteer_no,
            GROUP_CONCAT(g.id) AS group_ids_csv,
            GROUP_CONCAT(g.name) AS group_names_csv,
-           COALESCE((SELECT SUM(h.hours) FROM hours h WHERE h.user_id = u.id), 0) AS total_hours
+           COALESCE((
+             SELECT SUM(h.hours) FROM hours h
+              WHERE h.user_id = u.id
+                AND EXISTS (SELECT 1 FROM activities a WHERE a.id = h.activity_id)
+           ), 0) AS total_hours
     FROM users u
     LEFT JOIN user_groups ug ON u.id = ug.user_id
     LEFT JOIN work_groups g ON ug.group_id = g.id
@@ -66,7 +70,11 @@ router.get('/:id', (req: Request, res: Response) => {
     SELECT u.id, u.class, u.name, u.volunteer_no,
            GROUP_CONCAT(g.id) AS group_ids_csv,
            GROUP_CONCAT(g.name) AS group_names_csv,
-           COALESCE((SELECT SUM(h.hours) FROM hours h WHERE h.user_id = u.id), 0) AS total_hours
+           COALESCE((
+             SELECT SUM(h.hours) FROM hours h
+              WHERE h.user_id = u.id
+                AND EXISTS (SELECT 1 FROM activities a WHERE a.id = h.activity_id)
+           ), 0) AS total_hours_raw
     FROM users u
     LEFT JOIN user_groups ug ON u.id = ug.user_id
     LEFT JOIN work_groups g ON ug.group_id = g.id