fix: configure Opus encoder bitrate from available bandwidth

Replace SetBitrateToMax() with dynamic bitrate configuration based on
the per-frame byte budget (AudioDataBytes). Previously, the encoder
always targeted maximum bitrate (~510 kbps) and relied on output
truncation when frames exceeded the available bandwidth. This could
produce truncated/invalid Opus frames when the server's max bandwidth
is lower than the encoder's target.

The bitrate is now recalculated on every Encode() call as:
  maxDataBytes * 8 * 100 (bits per second for 10ms frames)

This ensures the encoder produces frames that fit within the byte
budget without truncation.

gumble/opus/opus.go

@@ -29,9 +29,8 @@ func (*generator) ID() int {
 func (*generator) NewEncoder() gumble.AudioEncoder {
 	// Force mono for voice transmission
 	e, _ := opus.NewEncoder(gumble.AudioSampleRate, VoiceChannels, opus.AppVoIP)
-	_ = e.SetBitrateToMax()
 	return &Encoder{
-		e,
+		Encoder: e,
 	}
 }
 
@@ -39,9 +38,8 @@ func (*generator) NewEncoder() gumble.AudioEncoder {
 func NewStereoEncoder() gumble.AudioEncoder {
 	// Create stereo encoder for file playback
 	e, _ := opus.NewEncoder(gumble.AudioSampleRate, gumble.AudioChannels, opus.AppAudio)
-	_ = e.SetBitrateToMax()
 	return &Encoder{
-		e,
+		Encoder: e,
 	}
 }
 
@@ -65,6 +63,13 @@ func (*Encoder) ID() int {
 }
 
 func (e *Encoder) Encode(pcm []int16, _, maxDataBytes int) ([]byte, error) {
+	// Set the encoder bitrate to match the available bandwidth per frame.
+	// The bitrate is: maxDataBytes * 8 (bits per byte) * 100 (10ms frames per second).
+	// Opus encodes best when the bitrate target is set properly rather than
+	// relying on truncation, which can produce incomplete or corrupt frames.
+	bitrate := maxDataBytes * 8 * 100
+	_ = e.Encoder.SetBitrate(bitrate)
+
 	buf := make([]byte, maxDataBytes)
 	n, err := e.Encoder.Encode(pcm, buf)
 	if err != nil {