Cache library filesystem scans to fix ~90s NAS page loads

There was no caching anywhere in the app, so every request re-walked
the SMB-mounted library from scratch - the dashboard alone triggered
2 duplicate full-library directory walks, 100+ individual
.offlineu_progress.json opens, and ~10-15 full per-course recursive
rglob() scans, none of it shared between requests. Measured ~90s per
dashboard load against the real mounted NAS, with a second immediate
reload taking just as long - proof nothing was being reused.

Add a minimal in-process TTL cache (5 min) and apply it at the actual
hot spots: the shared course-directory listing (get_all_course_dirs,
replacing 6 independent iter_all_courses() walks), the per-course
media-count/thumbnail summary (_course_summary, also now reused by
list_library_directory instead of a third duplicate implementation),
the per-course tree scan (get_course_tree - safe to cache since
progress is always re-applied fresh on top, never baked into the
cached structure), and the transcript-search subtitle index (the
worst offender - previously re-read and lowercased every subtitle
file in the library on every search).

Invalidates immediately on hide/show, rename, and library-path
changes; a "Refresh Library" button on Settings covers files added
directly on the NAS outside the app.

Measured after the fix, same real NAS mount: dashboard ~90s -> 1.3s
warm, Notes Hub ~1.15s warm, Library browser ~3.6s warm.

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

templates/settings.html

@@ -432,6 +432,11 @@
                     <button class="btn" onclick="saveLibraryPath()">Save</button>
                 </div>
                 <span id="library-path-status" style="font-size: 0.85em; min-height: 1.2em;"></span>
+                <div style="display: flex; align-items: center; gap: 10px;">
+                    <button class="btn btn-secondary btn-sm" onclick="refreshLibraryCache()">🔄 Refresh Library</button>
+                    <span id="library-refresh-status" style="font-size: 0.85em; color: var(--text-muted);"></span>
+                </div>
+                <span class="setting-desc">Courses are cached briefly for speed - use this after adding or removing files directly on disk if you don't want to wait a few minutes for it to notice.</span>
             </div>
         </div>
 
@@ -951,6 +956,23 @@
             });
         }
 
+        function refreshLibraryCache() {
+            const status = document.getElementById('library-refresh-status');
+            status.style.color = 'var(--text-muted)';
+            status.textContent = 'Refreshing…';
+            fetch('/api/library/refresh', { method: 'POST' })
+                .then(r => r.json())
+                .then(data => {
+                    status.style.color = data.success ? 'var(--success)' : 'var(--error)';
+                    status.textContent = data.success ? '✓ Refreshed' : 'Could not refresh';
+                    if (data.success) setTimeout(() => { status.textContent = ''; }, 3000);
+                })
+                .catch(() => {
+                    status.style.color = 'var(--error)';
+                    status.textContent = 'Could not reach the server';
+                });
+        }
+
         function loadCourseFromPath() {
             const input = document.getElementById('manual-course-path');
             const status = document.getElementById('manual-course-status');