fix: add ffmpeg to API runtime image for cover fallback

PR #1360 introduced FFmpeg-based frame extraction as fallback for cover
generation, but the API Docker image did not include FFmpeg binary.
Worker image already has it via worker-base-runtime, but API image only
had libpq5 in its runtime stage.

This caused subprocess calls to fail with FileNotFoundError when the
cover generation tried to use _extract_frames_with_ffmpeg fallback.

Fix: add ffmpeg to apt-get install in runtime stage of api.Dockerfile.

infra/docker/api.Dockerfile

@@ -53,9 +53,10 @@ ARG APP_VERSION=dev
 RUN sed -i 's|deb.debian.org|mirrors.aliyun.com|g' /etc/apt/sources.list.d/debian.sources 2>/dev/null || \
     sed -i 's|deb.debian.org|mirrors.aliyun.com|g' /etc/apt/sources.list 2>/dev/null || true
 
-# 只装运行时需要的库（libpq5 是 psycopg2 运行时依赖）
+# 只装运行时需要的库（libpq5 是 psycopg2 运行时依赖，ffmpeg 用于封面兜底取帧）
 RUN apt-get update && apt-get install -y --no-install-recommends \
     libpq5 \
+    ffmpeg \
     && rm -rf /var/lib/apt/lists/*
 
 # 从 builder 复制虚拟环境