fix: add ffmpeg to Docker image for video transcoding

Required for iOS MKV→MP4 on-the-fly transcoding.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Dockerfile

@@ -2,8 +2,8 @@ FROM python:3.11-slim
 
 WORKDIR /app
 
-# Install git for pip git+https dependencies
-RUN apt-get update && apt-get install -y --no-install-recommends git && rm -rf /var/lib/apt/lists/*
+# Install git and ffmpeg (for video transcoding)
+RUN apt-get update && apt-get install -y --no-install-recommends git ffmpeg && rm -rf /var/lib/apt/lists/*
 
 # Install dependencies
 COPY requirements.txt .