feat: add device traffic metrics with gateway telemetry reporting and admin UI display

Add rx_bytes and tx_bytes fields to Device type and API responses. Add formatDataSize helper for human-readable byte formatting with units from B to TB. Add Received and Sent columns to devices table in admin UI with formatted traffic totals. Add traffic metrics display to device action panel.

Add TelemetrySnapshot and PeerTelemetry types for gateway runtime stats. Add gateway telemetry endpoint at POST /gateway

backend/internal/device/repository.go

@@ -2,6 +2,7 @@ package device
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"net/netip"
 	"time"
@@ -198,7 +199,10 @@ func (r *PGRepository) ListByUser(ctx context.Context, userID uuid.UUID) ([]Devi
 		}
 		items = append(items, item)
 	}
-	return items, rows.Err()
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return r.applyRuntimeStats(ctx, items)
 }
 
 func (r *PGRepository) ListAll(ctx context.Context) ([]Device, error) {
@@ -222,7 +226,10 @@ func (r *PGRepository) ListAll(ctx context.Context) ([]Device, error) {
 		}
 		items = append(items, item)
 	}
-	return items, rows.Err()
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return r.applyRuntimeStats(ctx, items)
 }
 
 func (r *PGRepository) Revoke(ctx context.Context, deviceID uuid.UUID) error {
@@ -258,6 +265,69 @@ type enrollmentRowScanner interface {
 	Scan(dest ...any) error
 }
 
+type runtimeSnapshot struct {
+	Peers []runtimePeer `json:"peers"`
+}
+
+type runtimePeer struct {
+	DeviceID string `json:"device_id"`
+	RXBytes  uint64 `json:"rx_bytes"`
+	TXBytes  uint64 `json:"tx_bytes"`
+}
+
+func (r *PGRepository) applyRuntimeStats(ctx context.Context, items []Device) ([]Device, error) {
+	if len(items) == 0 {
+		return items, nil
+	}
+
+	rows, err := r.db.Query(ctx, `
+		select value
+		from settings
+		where category = 'gateway_runtime'
+	`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	statsByDevice := make(map[uuid.UUID]runtimePeer)
+	for rows.Next() {
+		var raw []byte
+		if err := rows.Scan(&raw); err != nil {
+			return nil, err
+		}
+
+		var snapshot runtimeSnapshot
+		if err := json.Unmarshal(raw, &snapshot); err != nil {
+			continue
+		}
+
+		for _, peer := range snapshot.Peers {
+			deviceID, err := uuid.Parse(peer.DeviceID)
+			if err != nil {
+				continue
+			}
+			existing := statsByDevice[deviceID]
+			existing.DeviceID = peer.DeviceID
+			existing.RXBytes += peer.RXBytes
+			existing.TXBytes += peer.TXBytes
+			statsByDevice[deviceID] = existing
+		}
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+
+	for index := range items {
+		if stats, ok := statsByDevice[items[index].ID]; ok {
+			items[index].RXBytes = stats.RXBytes
+			items[index].TXBytes = stats.TXBytes
+		}
+	}
+
+	return items, nil
+}
+
 func scanEnrollmentRow(row enrollmentRowScanner) (EnrollmentResponse, error) {
 	var response EnrollmentResponse
 	var profileRevision int

backend/internal/device/types.go

@@ -19,6 +19,8 @@ type Device struct {
 	Platform   string    `json:"platform"`
 	Status     string    `json:"status"`
 	AssignedIP string    `json:"assigned_ip,omitempty"`
+	RXBytes    uint64    `json:"rx_bytes"`
+	TXBytes    uint64    `json:"tx_bytes"`
 }
 
 type ConnectionStatus struct {