feat: add monitoring agent (Go) + agent report API + cloud billing UI

- Go agent: CPU/mem/disk/port collectors, HTTP report, static binary
- Cross-compile: linux/amd64 + linux/arm64 (statically linked)
- One-line install script for all Linux distros
- Backend: POST /api/v1/agent/report (no JWT) + GET /agent/latest,history (JWT)
- CloudBilling.tsx: fixed syntax errors, stats/trends/records/alerts
- K8s menu renamed to Kubernetes管理

Signed-off-by: Logan.Li

backend/internal/models/agent.go

@@ -0,0 +1,71 @@
+package models
+
+import (
+	"time"
+
+	"gorm.io/gorm"
+)
+
+// AgentReport Agent 上报的原始指标数据 // Logan.Li
+type AgentReport struct {
+	ID          uint           `gorm:"primarykey" json:"id"`
+	Hostname    string         `gorm:"size:255;index;not null" json:"hostname"`
+	CPUUsage    float64        `gorm:"type:decimal(6,2)" json:"cpu_usage"`     // CPU 使用率 %
+	CPUCores    int            `json:"cpu_cores"`                             // CPU 核心数
+	Load1       float64        `gorm:"type:decimal(8,2)" json:"load1"`        // 1分钟负载
+	Load5       float64        `gorm:"type:decimal(8,2)" json:"load5"`        // 5分钟负载
+	Load15      float64        `gorm:"type:decimal(8,2)" json:"load15"`       // 15分钟负载
+	MemTotal    float64        `gorm:"type:decimal(10,2)" json:"mem_total"`   // 总内存 GB
+	MemUsed     float64        `gorm:"type:decimal(10,2)" json:"mem_used"`    // 已用内存 GB
+	MemUsage    float64        `gorm:"type:decimal(6,2)" json:"mem_usage"`    // 内存使用率 %
+	DiskInfo    string         `gorm:"type:text" json:"disk_info"`            // 磁盘信息 JSON
+	PortInfo    string         `gorm:"type:text" json:"port_info"`            // 端口信息 JSON
+	ReportTime  time.Time      `json:"report_time"`                           // 上报时间
+	CreatedAt   time.Time      `json:"created_at"`
+	UpdatedAt   time.Time      `json:"updated_at"`
+	DeletedAt   gorm.DeletedAt `gorm:"index" json:"-"`
+}
+
+func (AgentReport) TableName() string {
+	return "agent_reports"
+}
+
+// AgentReportRequest Agent 上报请求体 // Logan.Li
+type AgentReportRequest struct {
+	Hostname  string  `json:"hostname"`
+	Timestamp string  `json:"timestamp"`
+	CPU       CPUMetric `json:"cpu"`
+	Memory    MemMetric `json:"memory"`
+	Disks     []DiskMetric `json:"disks"`
+	Ports     []PortMetric `json:"ports"`
+}
+
+type CPUMetric struct {
+	UsagePercent float64 `json:"usage_percent"`
+	CoreCount    int     `json:"core_count"`
+	Load1        float64 `json:"load1"`
+	Load5        float64 `json:"load5"`
+	Load15       float64 `json:"load15"`
+}
+
+type MemMetric struct {
+	TotalGB      float64 `json:"total_gb"`
+	UsedGB       float64 `json:"used_gb"`
+	CachedGB     float64 `json:"cached_gb"`
+	UsagePercent float64 `json:"usage_percent"`
+}
+
+type DiskMetric struct {
+	Mount        string  `json:"mount"`
+	Device       string  `json:"device"`
+	TotalGB      float64 `json:"total_gb"`
+	UsedGB       float64 `json:"used_gb"`
+	UsagePercent float64 `json:"usage_percent"`
+}
+
+type PortMetric struct {
+	Port     int    `json:"port"`
+	Protocol string `json:"protocol"`
+	Process  string `json:"process"`
+	PID      int    `json:"pid"`
+}