fix(media-server): use single sendrecv transceiver for Meta peer

The offer the media-server sent to Meta had two m=audio sections — one
sendonly (from AddTrack creating an implicit transceiver) plus one
recvonly (from an explicit AddTransceiverFromKind for outgoing calls).
Meta rejected every outbound call with error 100 \"Invalid parameter\"
at the terminate webhook, so the contact's phone never rang.

Bind the local audio track to a single sendrecv transceiver via
AddTransceiverFromTrack and drop the redundant recvonly transceiver.
The offer is now a single m=audio sendrecv line with BUNDLE 0, which is
what Meta expects.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

enterprise/media-server/internal/peer/meta_peer.go

@@ -88,11 +88,17 @@ func NewMetaPeer(cfg *config.Config, sdpOffer string, iceServers []webrtc.ICESer
 		return nil, "", fmt.Errorf("create local track: %w", err)
 	}
 
-	sender, err := pc.AddTrack(localTrack)
+	// Bind the local track to a single sendrecv transceiver. Using AddTrack +
+	// a separate recvonly transceiver yields two m=audio sections, which Meta
+	// rejects with error 100 "Invalid parameter".
+	transceiver, err := pc.AddTransceiverFromTrack(localTrack, webrtc.RTPTransceiverInit{
+		Direction: webrtc.RTPTransceiverDirectionSendrecv,
+	})
 	if err != nil {
 		pc.Close()
-		return nil, "", fmt.Errorf("add local track: %w", err)
+		return nil, "", fmt.Errorf("add audio transceiver: %w", err)
 	}
+	sender := transceiver.Sender()
 
 	// Consume RTCP packets from the sender to avoid blocking.
 	go func() {
@@ -167,15 +173,8 @@ func NewMetaPeer(cfg *config.Config, sdpOffer string, iceServers []webrtc.ICESer
 
 		sdpResult = pc.LocalDescription().SDP
 	} else {
-		// Outgoing call: we generate an offer to send to Meta.
-		// Add a recvonly transceiver so Meta knows we expect audio.
-		if _, err := pc.AddTransceiverFromKind(webrtc.RTPCodecTypeAudio, webrtc.RTPTransceiverInit{
-			Direction: webrtc.RTPTransceiverDirectionRecvonly,
-		}); err != nil {
-			pc.Close()
-			return nil, "", fmt.Errorf("add audio transceiver: %w", err)
-		}
-
+		// Outgoing call: we generate an offer to send to Meta. The sendrecv
+		// transceiver registered above already advertises both directions.
 		offer, err := pc.CreateOffer(nil)
 		if err != nil {
 			pc.Close()