新增车辆表及OTA权限与逻辑优化

新增车辆任务与报警表，完善实体与DbContext配置。细化OTA权限校验，增强回传会话IP安全。优化OTA上传与设置面板，调度器支持重启恢复。报警采集逻辑支持历史分段。

MiGu.DB/Domains/Fleet/VehicleAlarmRecord.cs

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+namespace MiGu.DB.Domains.Fleet;
+
+/// <summary>
+/// 车辆报警的平台侧记录（表 vehicle_alarms）。
+/// 出现报警→开一条 active 记录；文案/级别变化→先 clear 再建新 active；消失→cleared。
+/// </summary>
+public sealed class VehicleAlarmRecord
+{
+    public string Id { get; set; } = Guid.NewGuid().ToString("D");
+    public int CarId { get; set; }
+    public string CarName { get; set; } = "";
+    /// <summary>报警文案（车体_AlarmInfo）。</summary>
+    public string Info { get; set; } = "";
+    /// <summary>报警级别（车体_AlarmLevel，未知为 0）。</summary>
+    public int Level { get; set; }
+    /// <summary>active | cleared</summary>
+    public string Status { get; set; } = "active";
+    public DateTimeOffset FirstAt { get; set; }
+    public DateTimeOffset LastAt { get; set; }
+    public DateTimeOffset? ResolvedAt { get; set; }
+    /// <summary>持续时长（秒），恢复后写入。</summary>
+    public long? DurationSecs { get; set; }
+    /// <summary>预留：平台侧确认（不代表车端消警）。</summary>
+    public bool Acknowledged { get; set; }
+    public DateTimeOffset? AcknowledgedAt { get; set; }
+    public string? AcknowledgedBy { get; set; }
+}