feat: switch to nvidia/cuda base image for NVENC hw encoding

- Base: nvidia/cuda:12.6.3-runtime-ubuntu24.04
- ffmpeg from apt has NVENC support when GPU runtime is available
- docker-compose reserves all GPUs via deploy.resources

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

Dockerfile

@@ -1,8 +1,13 @@
-FROM python:3.12-slim
-RUN apt-get update && apt-get install -y ffmpeg && rm -rf /var/lib/apt/lists/*
+FROM nvidia/cuda:12.6.3-runtime-ubuntu24.04
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        python3 python3-pip ffmpeg \
+    && rm -rf /var/lib/apt/lists/*
+
 WORKDIR /app
 COPY core/ core/
 COPY server/ server/
-RUN pip install --no-cache-dir fastapi uvicorn[standard]
+RUN pip install --no-cache-dir --break-system-packages fastapi uvicorn[standard]
+
 EXPOSE 8000
 CMD ["uvicorn", "server.app:app", "--host", "0.0.0.0", "--port", "8000"]