feat: migrate to typed messages, drop base64, fix AudioBuffer

- AudioBuffer getAudio(): use ab.totalBytes directly (eliminates triple-copy)
- Decode STTStreamMessage via natsutil.Decode[messages.STTStreamMessage]
- Audio chunks arrive as raw []byte (no base64 decode needed)
- Publish STTTranscription struct (not map[string]any)
- Interrupts use messages.STTInterrupt
- Remove encoding/base64 import
- Add .dockerignore, GOAMD64=v3 in Dockerfile
- All 15 tests pass

Dockerfile

@@ -10,7 +10,7 @@ RUN go mod download
 
 COPY . .
 
-RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-w -s" -o /stt-module .
+RUN CGO_ENABLED=0 GOOS=linux GOAMD64=v3 go build -ldflags="-w -s" -o /stt-module .
 
 # Runtime stage
 FROM scratch