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/agent.go

@@ -76,12 +76,23 @@ func (h *AgentHandler) ReceiveReport(c *gin.Context) {
 
 	// 同步更新 servers 表的实时指标（通过 hostname 匹配）
 	diskAvg := avgDiskUsage(req.Disks)
+
+	// 计算硬件规格
+	totalDiskGB := 0.0
+	for _, d := range req.Disks {
+		totalDiskGB += d.TotalGB
+	}
+	memMB := int64(req.Memory.TotalGB * 1024) // GB → MB
+
 	result := h.db.Model(&models.Server{}).
 		Where("hostname = ? OR ip = ?", req.Hostname, req.Hostname).
 		Updates(map[string]interface{}{
 			"cpu_usage":    req.CPU.UsagePercent,
 			"memory_usage": req.Memory.UsagePercent,
 			"disk_usage":   diskAvg,
+			"cpu":          req.CPU.CoreCount,
+			"memory":       memMB,
+			"disk":         int64(totalDiskGB),
 			"status":       "running",
 		})