feat: Agent采集uptime, 后端持久化kernel/uptime/diskIO, 前端磁盘BPS+IOPS展示

- Agent: system.go 添加 readUptime(), SystemStat 携带 Uptime
- Backend: Server 模型新增 Kernel 字段, ReceiveReport 持久化 kernel+uptime
- Backend: /agent/latest 返回 kernel/uptime/port_info/disk_io_info/disk_total
- Frontend: 磁盘监控改为 BPS+IOPS (4张卡片), 系统信息修复内核/运行时长/监听端口/磁盘容量
- 图表单位从 MB/s 改为 KB/s, 添加 formatBPS 工具函数

backend/internal/api/handlers/credential_assign.go

@@ -0,0 +1,62 @@
+package handlers
+
+import (
+	"net/http"
+	"strconv"
+
+	"ezops/internal/models"
+
+	"github.com/gin-gonic/gin"
+)
+
+// AssignCredential 将凭据分配给服务器（解密凭据并写入 password/ssh_key） // Logan.Li
+func (h *CredentialHandler) AssignCredential(c *gin.Context) {
+	serverID, err := strconv.ParseUint(c.Param("id"), 10, 32)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, models.Response{Code: 400, Message: "无效的服务器ID"})
+		return
+	}
+
+	var req struct {
+		CredentialID uint `json:"credential_id"`
+	}
+	if err := c.ShouldBindJSON(&req); err != nil || req.CredentialID == 0 {
+		c.JSON(http.StatusBadRequest, models.Response{Code: 400, Message: "请选择凭据"})
+		return
+	}
+
+	var cred models.Credential
+	if err := h.db.First(&cred, req.CredentialID).Error; err != nil {
+		c.JSON(http.StatusNotFound, models.Response{Code: 404, Message: "凭据不存在"})
+		return
+	}
+
+	plain, err := h.crypt.Decrypt(cred.Content)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, models.Response{Code: 500, Message: "凭据解密失败"})
+		return
+	}
+
+	var server models.Server
+	if err := h.db.First(&server, serverID).Error; err != nil {
+		c.JSON(http.StatusNotFound, models.Response{Code: 404, Message: "服务器不存在"})
+		return
+	}
+
+	if cred.Type == "ssh_key" {
+		server.SSHKey = plain
+		server.AuthType = "key"
+		server.Password = ""
+	} else {
+		server.Password = plain
+		server.AuthType = "password"
+		server.SSHKey = ""
+	}
+
+	if err := h.db.Save(&server).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, models.Response{Code: 500, Message: "分配失败: " + err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusOK, models.Response{Code: 200, Message: "凭据已关联"})
+}