fix(admin-service): 修复 getCurrentStatus 方法使用旧响应格式的问题

AdminMaintenanceController.getCurrentStatus() 方法仍使用旧的 inMaintenance 字段，
导致构建失败。更新为与 MobileMaintenanceController 一致的新格式：
- 使用 isUnderMaintenance 代替 inMaintenance
- 使用嵌套的 maintenance 对象包含详情
- 添加 remainingMinutes 字段计算

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/admin-service/src/api/controllers/system-maintenance.controller.ts

@@ -176,14 +176,24 @@ export class AdminMaintenanceController {
     const active = await this.maintenanceRepo.findActiveMaintenance();
 
     if (!active) {
-      return { inMaintenance: false };
+      return { isUnderMaintenance: false, maintenance: null };
     }
 
+    // 计算剩余分钟数
+    const now = new Date();
+    const endTime = new Date(active.endTime);
+    const remainingMs = endTime.getTime() - now.getTime();
+    const remainingMinutes = Math.max(0, Math.ceil(remainingMs / 60000));
+
     return {
-      inMaintenance: true,
-      title: active.title,
-      message: active.message,
-      endTime: active.endTime,
+      isUnderMaintenance: true,
+      maintenance: {
+        title: active.title,
+        message: active.message,
+        startTime: active.startTime,
+        endTime: active.endTime,
+        remainingMinutes,
+      },
     };
   }
 }