Add per-episode play tracking with bar chart in admin analytics

- POST /api/analytics/play records a play event (title + date) on first
  play per player mount, skipping admin sessions
- Plays persisted to data/episode-plays.json with total + byDay buckets
- Admin stats response includes episodePlays sorted by total plays
- AnalyticsPanel shows horizontal bar chart + summary cards for all
  episodes dynamically — new episodes appear automatically as played

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server.js

@@ -20,6 +20,7 @@ import {
   migrateStudyNotesIfNeeded,
   loadDownloadCountsFromDisk,
   loadQrCodesFromDisk,
+  loadEpisodePlaysFromDisk,
   loadPodcastChecklistFromDisk,
   createBackupSnapshot,
   refreshContentCaches,
@@ -112,6 +113,7 @@ Promise.all([
   migrateStudyNotesIfNeeded(),
   loadDownloadCountsFromDisk(),
   loadQrCodesFromDisk(),
+  loadEpisodePlaysFromDisk(),
   loadPodcastChecklistFromDisk(),
   refreshContentCaches(),
 ])