feat: 修复服务器硬件规格(CPU核/内存/磁盘) + Top 5 资源排行

修复:
- agent.go ReceiveReport 写入 cpu(核数)/memory(总内存MB)/disk(总磁盘GB) 到 servers 表
- 之前 Agent 上报只写了 cpu_usage/memory_usage/disk_usage 使用率，硬件规格始终为 0

新功能:
- GET /api/v1/metrics/top?metric=cpu|memory|disk&limit=5 主机排行 API
- HostMonitor 新增 Top 5 排行面板: CPU/内存/磁盘 使用率排名
- TopRankCard 组件: 排名号+主机名+进度条+百分比

backend/internal/api/handlers/metrics.go

@@ -127,6 +127,62 @@ func (h *MetricsHandler) ListMetricNames(c *gin.Context) {
 	c.JSON(http.StatusOK, models.Response{Code: 200, Data: names})
 }
 
+// TopHosts 返回指定指标的 Top N 主机排行
+// GET /api/v1/metrics/top?metric=cpu&limit=5
+func (h *MetricsHandler) TopHosts(c *gin.Context) {
+	metric := c.DefaultQuery("metric", "cpu")
+	limit, _ := strconv.Atoi(c.DefaultQuery("limit", "5"))
+
+	type HostRank struct {
+		Hostname string  `json:"hostname"`
+		IP       string  `json:"ip"`
+		CPUUsage float64 `json:"cpu_usage"`
+		MemUsage float64 `json:"memory_usage"`
+		DiskUsage float64 `json:"disk_usage"`
+		CPUCores int    `json:"cpu_cores"`
+		MemMB    int64  `json:"memory_mb"`
+		DiskGB   int64  `json:"disk_gb"`
+		Status   string `json:"status"`
+	}
+
+	var servers []models.Server
+	orderField := "cpu_usage DESC"
+	switch metric {
+	case "cpu":
+		orderField = "cpu_usage DESC"
+	case "mem", "memory":
+		orderField = "memory_usage DESC"
+	case "disk":
+		orderField = "disk_usage DESC"
+	default:
+		orderField = "cpu_usage DESC"
+	}
+
+	h.db.Where("agent_status = ?", "online").
+		Order(orderField).
+		Limit(limit).
+		Find(&servers)
+
+	result := make([]HostRank, len(servers))
+	for i, s := range servers {
+		result[i] = HostRank{
+			Hostname: s.Hostname,
+			IP:       s.IP,
+			CPUUsage: s.CPUUsage,
+			MemUsage: s.MemoryUsage,
+			DiskUsage: s.DiskUsage,
+			CPUCores: s.CPU,
+			MemMB:    s.Memory,
+			DiskGB:   s.Disk,
+			Status:   s.Status,
+		}
+	}
+	if result == nil {
+		result = []HostRank{}
+	}
+	c.JSON(http.StatusOK, models.Response{Code: 200, Data: result})
+}
+
 func now() int64 {
 	return time.Now().Unix()
 }