Show course runtime on library cards via ffprobe

Add ffmpeg to the Docker image so ffprobe can read each video/audio
file's duration server-side. Results are cached to a small persistent
per-course JSON file keyed by (relative path, size, mtime), so a file
only gets probed once - a container restart or the library scan's
in-memory cache expiring never re-runs ffprobe on unchanged files.

Course cards in the library browser (grid, list, and search) now show
total runtime alongside the media-file count, so you know the time
commitment before opening a course. Durations under a minute are
suppressed rather than showing a noisy "0m".

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Dockerfile

@@ -4,6 +4,11 @@ FROM python:3.13.5-slim-bookworm
 # set the working directory in the container
 WORKDIR /app
 
+# ffprobe (from ffmpeg) reads each video's duration for the library
+# browser's "how long is this course" display
+RUN apt-get update && apt-get install -y --no-install-recommends ffmpeg \
+    && rm -rf /var/lib/apt/lists/*
+
 # copy the dependencies file to the working directory
 COPY requirements.txt .