fix(activity): 参与人昵称/头像回填真实用户，已注销用户明确标注

- getActivities/getActivityById 的 participants 关联 users，返回真实昵称与头像

- 用户已删除时返回「用户已注销」，避免详情页显示搭子_id

- 118 套件/773 用例全绿；线上活动详情已验证真实昵称

server/src/db/repository.ts

@@ -174,7 +174,7 @@ export class MysqlRepository {
           id: p.id,
           activityId: p.activityId,
           userId: p.userId,
-          nickname: p.nickname || '',
+          nickname: p.nickname || '用户已注销',
           avatar: p.avatar || '',
           payAmountCents: p.payAmountCents,
           checkinStatus: Boolean(p.checkinStatus),
@@ -238,7 +238,11 @@ export class MysqlRepository {
     return {
       ...act,
       allowDynamicPrice: Boolean(act.allowDynamicPrice),
-      participants: ptRows.map((p: any) => ({ ...p, checkinStatus: Boolean(p.checkinStatus) })),
+      participants: ptRows.map((p: any) => ({
+        ...p,
+        nickname: p.nickname || '用户已注销',
+        checkinStatus: Boolean(p.checkinStatus),
+      })),
       joinedCount: ptRows.length,
     };
   }