Add build-essential to Dockerfile for webrtcvad compilation

Dockerfile

@@ -8,9 +8,11 @@ WORKDIR /app
 # Install system dependencies for audio processing
 # ffmpeg: for audio format conversion
 # libsndfile1: for soundfile library to read/write audio files
+# build-essential: for compiling webrtcvad (includes gcc, g++, make)
 RUN apt-get update && apt-get install -y \
     ffmpeg \
     libsndfile1 \
+    build-essential \
     && rm -rf /var/lib/apt/lists/*
 
 # Copy requirements file first (for better Docker layer caching)