fix: add pkg-config and FFmpeg dev libs for PyAV build

PyAV (av==11, dep of faster-whisper) requires pkg-config and
FFmpeg development headers to compile from source.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/services/voice-service/Dockerfile

@@ -7,6 +7,13 @@ RUN apt-get update && apt-get install -y \
     ffmpeg \
     libsndfile1 \
     build-essential \
+    pkg-config \
+    libavformat-dev \
+    libavcodec-dev \
+    libavdevice-dev \
+    libavutil-dev \
+    libswscale-dev \
+    libswresample-dev \
     && rm -rf /var/lib/apt/lists/*
 
 COPY requirements.txt .