fix: guard gateway & voice shared state against data races

Summary

Mutable fields on gatewayImpl (HTTP & voice) and the audio sender/receiver were read/written from multiple goroutines (listen loop, heartbeat, and caller Open/Close/Latency) without synchronization. This adds mutexes and accessor helpers so all shared state is accessed safely, fixing the races flagged under -race.

Changes

• gateway/gateway.go — heartbeatMu guards heartbeatCancel + heartbeat timestamps; seqMu guards LastSequenceReceived. setLastSequenceReceived now stores a copy, so the pointer no longer aliases the listen loop's message struct.
• voice/gateway.go — same heartbeat/seq guarding, plus lastNonce, ssrc, and seq accessors.
• voice/audio_sender.go / audio_receiver.go — sync.Mutex guards cancelFunc, now created in Open() before the goroutine starts and nil-checked in Close() (fixes a race + possible nil deref).

Testing

• go test --race -cover ./...
• go build ./...
• golangci-lint run