feat: initial arena-dashboard (FastAPI + dark UI + liveness probe)

Dockerfile

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+FROM python:3.12-slim
+
+WORKDIR /app
+
+RUN pip install --no-cache-dir fastapi uvicorn[standard] httpx
+
+COPY . /app/
+
+# cron для probe-скрипта
+RUN apt-get update -qq && apt-get install -y -qq cron && rm -rf /var/lib/apt/lists/*
+
+# entrypoint запускает cron + uvicorn
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+
+EXPOSE 8000
+
+ENTRYPOINT ["/entrypoint.sh"]