fix: 设备变更时同步清除用户缓存，修复 user_devices 为空问题

Device.GetCacheKeys() 新增 cacheUserIdPrefix+UserId，
确保 InsertDevice/UpdateDevice/DeleteDevice 时同时失效
cache:user:id:{userId}，避免下次 FindOne 读到不含
UserDevices 的旧缓存。

Co-Authored-By: claude-flow <ruv@ruv.net>

internal/model/user/cache.go

@@ -102,6 +102,9 @@ func (d *Device) GetCacheKeys() []string {
 	if d.Identifier != "" {
 		keys = append(keys, fmt.Sprintf("%s%s", cacheUserDeviceNumberPrefix, d.Identifier))
 	}
+	if d.UserId != 0 {
+		keys = append(keys, fmt.Sprintf("%s%d", cacheUserIdPrefix, d.UserId))
+	}
 	return keys
 }