fix: add minimum Opus bitrate floor in encoder

Add a defensive floor of 8000 bps in the Encode method, preventing the
encoder from being configured with an invalidly low bitrate even if
AudioDataBytes is somehow set to a very small value.

gumble/opus/opus.go

@@ -68,6 +68,9 @@ func (e *Encoder) Encode(pcm []int16, _, maxDataBytes int) ([]byte, error) {
 	// 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
+	if bitrate < 8000 {
+		bitrate = 8000 // Opus minimum viable bitrate for voice
+	}
 	_ = e.Encoder.SetBitrate(bitrate)
 
 	buf := make([]byte, maxDataBytes)