fix(monitoring): read LXC CPU, swap and uptime from the container (#6920)

In an LXC container with LXCFS the Overview showed the container's RAM
beside the host's swap, the host's CPU topology and a CPU percentage from
the host's view. Swap and uptime now come from LXCFS's virtualised files
and CPU from the container's cgroup v2 counter, normalised by the visible
vCPUs and any quota; container CPU and swap history is kept apart from
the earlier host samples.

A value the reader cannot measure (the first CPU sample, or a layout it
cannot verify such as cgroup v1) is reported as unavailable instead of 0:
the dashboard, the node list, node CPU history and the Telegram and
Discord uptime show a dash, and nodes keep the flags in a new
unavailable_metrics column.

internal/database/model/model.go

@@ -864,9 +864,11 @@ type Node struct {
 	CpuPct        float64 `json:"cpuPct" example:"23.5"`
 	MemPct        float64 `json:"memPct" example:"45.1"`
 	UptimeSecs    uint64  `json:"uptimeSecs" example:"86400"`
-	NetUp         uint64  `json:"netUp" gorm:"column:net_up" example:"1048576"`
-	NetDown       uint64  `json:"netDown" gorm:"column:net_down" example:"2097152"`
-	LastError     string  `json:"lastError"`
+	// Nil on legacy rows and responses: zero remains a valid measured value.
+	UnavailableMetrics []string `json:"unavailableMetrics,omitempty" gorm:"serializer:json;column:unavailable_metrics"`
+	NetUp              uint64   `json:"netUp" gorm:"column:net_up" example:"1048576"`
+	NetDown            uint64   `json:"netDown" gorm:"column:net_down" example:"2097152"`
+	LastError          string   `json:"lastError"`
 
 	// XrayState and XrayError are captured from the remote node's /panel/api/server/status
 	// during heartbeats. They let the central panel distinguish "panel API reachable"