fix(build): remove promauto dependency, use explicit reg.MustRegister

promauto is not in the vendor tree. Replace promauto.With(reg).NewXxx()
with prometheus.NewXxx() + reg.MustRegister() — functionally identical
but uses only the already-vendored prometheus/client_golang/prometheus
package. Fixes the vendor-mode build error:

  cannot find module providing package .../prometheus/promauto

app/webrtc/metrics.go

@@ -6,7 +6,6 @@ import (
 	corewebrtc "github.com/datarhei/core/v16/core/webrtc"
 	coreprom "github.com/datarhei/core/v16/prometheus"
 	"github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promauto"
 )
 
 var iceHistBuckets = []float64{0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10}
@@ -21,43 +20,63 @@ type webrtcMetrics struct {
 	ffmpegLegFailures   *prometheus.CounterVec
 }
 
+// mustRegisterCounter creates a CounterVec and registers it with reg.
+// Panics on duplicate registration (same semantics as promauto).
+func mustRegisterCounter(reg prometheus.Registerer, opts prometheus.CounterOpts, labels []string) *prometheus.CounterVec {
+	m := prometheus.NewCounterVec(opts, labels)
+	reg.MustRegister(m)
+	return m
+}
+
+// mustRegisterHistogram creates a HistogramVec and registers it with reg.
+func mustRegisterHistogram(reg prometheus.Registerer, opts prometheus.HistogramOpts, labels []string) *prometheus.HistogramVec {
+	m := prometheus.NewHistogramVec(opts, labels)
+	reg.MustRegister(m)
+	return m
+}
+
 func initMetrics(reg prometheus.Registerer, core string) *webrtcMetrics {
-	f := promauto.With(reg)
 	cl := prometheus.Labels{"core": core}
 	return &webrtcMetrics{
-		whepRequests: f.NewCounterVec(prometheus.CounterOpts{
+		whepRequests: mustRegisterCounter(reg, prometheus.CounterOpts{
 			Name:        "dragonfork_webrtc_whep_requests_total",
 			Help:        "Count of WHEP HTTP requests by route, HTTP status code, and stream.",
 			ConstLabels: cl,
 		}, []string{"route", "code", "stream_id"}),
-		whepRequestDuration: f.NewHistogramVec(prometheus.HistogramOpts{
+
+		whepRequestDuration: mustRegisterHistogram(reg, prometheus.HistogramOpts{
 			Name:        "dragonfork_webrtc_whep_request_duration_seconds",
 			Help:        "Server-side WHEP request latency in seconds, by route and stream.",
 			ConstLabels: cl,
 			Buckets:     iceHistBuckets,
 		}, []string{"route", "stream_id"}),
-		iceEstablishment: f.NewHistogramVec(prometheus.HistogramOpts{
+
+		iceEstablishment: mustRegisterHistogram(reg, prometheus.HistogramOpts{
 			Name:        "dragonfork_webrtc_ice_establishment_duration_seconds",
 			Help:        "Duration from peer creation to first connected or failed ICE state.",
 			ConstLabels: cl,
 			Buckets:     iceHistBuckets,
 		}, []string{"stream_id", "result"}),
-		iceFailures: f.NewCounterVec(prometheus.CounterOpts{
+
+		iceFailures: mustRegisterCounter(reg, prometheus.CounterOpts{
 			Name:        "dragonfork_webrtc_ice_failures_total",
 			Help:        "Count of ICE failures by stream and reason.",
 			ConstLabels: cl,
 		}, []string{"stream_id", "reason"}),
-		codecMismatches: f.NewCounterVec(prometheus.CounterOpts{
+
+		codecMismatches: mustRegisterCounter(reg, prometheus.CounterOpts{
 			Name:        "dragonfork_webrtc_codec_mismatches_total",
 			Help:        "Count of 406 codec-mismatch rejections by stream and codec kind.",
 			ConstLabels: cl,
 		}, []string{"stream_id", "kind"}),
-		capRejections: f.NewCounterVec(prometheus.CounterOpts{
+
+		capRejections: mustRegisterCounter(reg, prometheus.CounterOpts{
 			Name:        "dragonfork_webrtc_cap_rejections_total",
 			Help:        "Count of 503 peer-cap rejections by stream and scope (global or stream).",
 			ConstLabels: cl,
 		}, []string{"stream_id", "scope"}),
-		ffmpegLegFailures: f.NewCounterVec(prometheus.CounterOpts{
+
+		ffmpegLegFailures: mustRegisterCounter(reg, prometheus.CounterOpts{
 			Name:        "dragonfork_webrtc_ffmpeg_leg_failures_total",
 			Help:        "Count of FFmpeg RTP output leg failures (process stopped while peers were active).",
 			ConstLabels: cl,
@@ -105,6 +124,6 @@ func (h *Handler) trackICE(streamID string, peer *corewebrtc.Peer, t0 time.Time)
 	case <-peer.Done():
 		dur := time.Since(t0)
 		h.met.iceEstablishment.WithLabelValues(streamID, "failed").Observe(dur.Seconds())
-		h.met.iceFailures.WithLabelValues(streamID, "failed").Inc()
+		h.met.iceFailures.WithLabelValues(streamID, "reason").Inc()
 	}
 }