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>

templates/course_dashboard.html

@@ -1644,12 +1644,15 @@
             const progressBar = pct
                 ? `<div class="grid-card-progress-track"><div class="grid-card-progress-fill" style="width: ${pct}%;"></div></div>`
                 : '';
+            const metaBits = [`${item.media_files} media file${item.media_files === 1 ? '' : 's'}`];
+            if (item.duration_display) metaBits.push(item.duration_display);
+            if (pct) metaBits.push(`${pct}% done`);
             return `
                 <div class="library-grid-card" onclick="${clickHandler}">
                     ${selectableAttrs(item)}
                     <div class="grid-card-thumb-wrap">${iconHtml}</div>
                     <div class="grid-card-name" title="${escapeAttr(item.name)}">${item.name}</div>
-                    <div class="grid-card-meta">${item.media_files} media file${item.media_files === 1 ? '' : 's'}${pct ? ` · ${pct}% done` : ''}</div>
+                    <div class="grid-card-meta">${metaBits.join(' · ')}</div>
                     ${progressBar}
                 </div>
             `;
@@ -1688,7 +1691,7 @@
                     </div>
                     <div class="lesson-meta">
                         ${progressBadge}
-                        <span>${item.media_files} media file${item.media_files === 1 ? '' : 's'}</span>
+                        <span>${item.media_files} media file${item.media_files === 1 ? '' : 's'}${item.duration_display ? ` · ${item.duration_display}` : ''}</span>
                         <button class="btn btn-secondary btn-sm" onclick="event.stopPropagation(); queueCourse('${safePath}', this)" title="Add to Next Up">${ICON_SVGS.pin}</button>
                     </div>
                     ${progressBar}