feat(docker): add Dockerfile, docker-compose and headless fallback for local testing

Dockerfile

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+FROM python:3.12-slim-bookworm
+
+ENV DEBIAN_FRONTEND=noninteractive
+ENV PYTHONUNBUFFERED=1
+
+# Install mpv, ffmpeg and system dependencies
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    mpv \
+    ffmpeg \
+    curl \
+    ca-certificates \
+    libgl1 \
+    libegl1 \
+    libx11-6 \
+    libxext6 \
+    libasound2 \
+    pulseaudio-utils \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+
+# Install python dependencies and latest yt-dlp
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt yt-dlp
+
+COPY src/ ./src/
+COPY config.example.yaml ./config.example.yaml
+
+# If no config.yaml exists at runtime, fallback to config.example.yaml
+RUN cp config.example.yaml config.yaml
+
+EXPOSE 8080 9090 8097
+
+ENTRYPOINT ["python3", "-m", "src.main"]
+CMD ["-c", "config.yaml"]