feat(telemetry): agent 心跳遥测上报+节点列表覆写(第13条)

agent 每 60s 心跳捎带遥测: agentVersion/xrayVersion(按 mtime 缓存重读)/
uptime/最近一次主控请求 RTT/cpu%/mem%/公网 v4+v6(公网优先, ULA/私网兜底);
主控心跳处理按指针语义只存上报字段, 旧 agent 无遥测行为不变;
agent_nodes 加法迁移存遥测, 节点列表对 agent 行覆写真值,
panelVersion 显示 "agent <版本>", NodeView 新增 ipv4/ipv6。

internal/database/model/agent_node.go

@@ -22,4 +22,20 @@ type AgentNode struct {
 	LastSeen     int64  `json:"lastSeen"`
 	CreatedAt    int64  `json:"createdAt" gorm:"autoCreateTime:milli"`
 	UpdatedAt    int64  `json:"updatedAt" gorm:"autoUpdateTime:milli"`
+
+	// Heartbeat telemetry (P13): the agent piggybacks host/runtime facts on
+	// its 60s heartbeat and the panel overlays them onto the node list row.
+	// All fields are best-effort: an agent that never reported telemetry has
+	// TelemetryAt == 0 and the UI keeps showing "-" for its row.
+	AgentVersion string  `json:"agentVersion"`
+	XrayVersion  string  `json:"xrayVersion"`
+	UptimeSecs   uint64  `json:"uptimeSecs"`
+	LatencyMs    int     `json:"latencyMs"`
+	CpuPct       float64 `json:"cpuPct"`
+	MemPct       float64 `json:"memPct"`
+	IPv4         string  `json:"ipv4"`
+	IPv6         string  `json:"ipv6"`
+	// TelemetryAt is the unix-milli time of the last heartbeat that carried
+	// a telemetry object.
+	TelemetryAt int64 `json:"telemetryAt"`
 }