fix(stream): downmix estéreo en el audio re-encodeado del modo copy

Sin -ac 2 una fuente 5.1 (AC3/EAC3) producía AAC de 6 canales del encoder
nativo de ffmpeg, que WebKit/Apple HLS rechaza al sniffar el primer
segmento: en el access log de Safari se ve master → index → init → seg-0
dos veces y silencio. Era el discriminador exacto del patrón de campo:
episodios con AAC estéreo (copy de audio) reproducían en iPhone; todas las
películas 5.1 fallaban. Verificado con Safari/macOS via WebDriver-less
access log: con -ac 2 la progresión de segmentos avanza con normalidad.

Espeja los flags del path de encode (aac 192k 48kHz estéreo). Test smoke
ampliado: el re-encode debe llevar -ac 2.

internal/engine/hls.go

@@ -1968,7 +1968,12 @@ func buildHLSCopyArgs(cfg HLSSessionConfig, probe *StreamProbe, tmpDir string) [
 	if strings.EqualFold(audioCodec, "aac") {
 		args = append(args, "-c:a", "copy")
 	} else {
-		args = append(args, "-c:a", "aac", "-b:a", "192k")
+		// Mirror the encode path exactly: AAC stereo 48k. WITHOUT -ac 2 a 5.1
+		// source produces 6-channel ffmpeg-native AAC, which WebKit/Apple HLS
+		// rejects at the first media segment (observed via Safari access log:
+		// master → index → init → seg-0 fetched twice, then silence — every
+		// 5.1 movie failed on iPhone while stereo-AAC episodes played).
+		args = append(args, "-c:a", "aac", "-b:a", "192k", "-ar", "48000", "-ac", "2")
 	}
 
 	args = append(args,

internal/engine/hls_copy_smoke_test.go

@@ -183,6 +183,12 @@ func TestHLSCopy_H264Ac3TranscodesAudio(t *testing.T) {
 	if !containsSeq(args, "-c:a", "aac") {
 		t.Errorf("expected -c:a aac for AC3 source, args: %v", args)
 	}
+	// MUST downmix to stereo: 6-channel ffmpeg-native AAC is rejected by
+	// WebKit/Apple HLS at the first media segment (every 5.1 movie failed on
+	// iPhone while stereo-AAC sources played — confirmed via Safari access log).
+	if !containsSeq(args, "-ac", "2") {
+		t.Errorf("expected -ac 2 (stereo downmix) for re-encoded audio, args: %v", args)
+	}
 }
 
 func TestHLSCopy_Hevc10Eac3_IncidentShape(t *testing.T) {

internal/engine/stream_server.go

@@ -690,6 +690,14 @@ func (ss *StreamServer) HLSURLsJSON(sessionID string) string {
 func (ss *StreamServer) hlsHandler(w http.ResponseWriter, r *http.Request) {
 	ss.lastActivity.Store(time.Now().UnixNano())
 
+	// Debug access log (UNARR_HLS_DEBUG=1): which client fetches which HLS
+	// resource. Off by default — a player polls the playlist every few
+	// seconds and segments stream constantly, far too chatty for normal logs.
+	if os.Getenv("UNARR_HLS_DEBUG") == "1" {
+		host, _, _ := net.SplitHostPort(r.RemoteAddr)
+		log.Printf("[hls-debug] %s %s from %s UA=%q", r.Method, r.URL.Path, host, r.Header.Get("User-Agent"))
+	}
+
 	if ss.writeCORSHeaders(w, r, "Content-Length, Content-Range, Accept-Ranges") {
 		return
 	}