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/gateway/repository.go

@@ -2,6 +2,7 @@ package gateway
 
 import (
 	"context"
+	"encoding/json"
 	"net/netip"
 
 	"github.com/google/uuid"
@@ -14,6 +15,7 @@ type Repository interface {
 	List(ctx context.Context) ([]Gateway, error)
 	FirstActive(ctx context.Context) (Gateway, error)
 	BuildSyncBundle(ctx context.Context, gatewayID uuid.UUID) (wireguard.GatewayBundle, error)
+	StoreTelemetry(ctx context.Context, gatewayID uuid.UUID, snapshot TelemetrySnapshot) error
 	Update(ctx context.Context, gatewayID uuid.UUID, input UpdateRequest) (Gateway, error)
 	UpsertByName(ctx context.Context, input BootstrapRequest) (Gateway, error)
 }
@@ -139,6 +141,21 @@ func (r *PGRepository) Update(ctx context.Context, gatewayID uuid.UUID, input Up
 	return item, err
 }
 
+func (r *PGRepository) StoreTelemetry(ctx context.Context, gatewayID uuid.UUID, snapshot TelemetrySnapshot) error {
+	payload, err := json.Marshal(snapshot)
+	if err != nil {
+		return err
+	}
+
+	_, err = r.db.Exec(ctx, `
+		insert into settings (category, key, value, updated_at)
+		values ('gateway_runtime', $1, $2::jsonb, now())
+		on conflict (category, key)
+		do update set value = excluded.value, updated_at = now()
+	`, gatewayID.String(), string(payload))
+	return err
+}
+
 func (r *PGRepository) UpsertByName(ctx context.Context, input BootstrapRequest) (Gateway, error) {
 	row := r.db.QueryRow(ctx, `
 		insert into gateways (id, name, endpoint, public_key, listen_port, vpn_cidr, dns_servers, is_active)