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/state.js

@@ -59,6 +59,10 @@ export const state = {
   downloadCounts: {},
   downloadCountsWritePromise: Promise.resolve(),
 
+  // episodePlays: { [title]: { total: number, byDay: { [YYYY-MM-DD]: number } } }
+  episodePlays: {},
+  episodePlaysWritePromise: Promise.resolve(),
+
   // qrCodes: Array<{ id, slug, label, destination, createdAt }>
   // qrScans: Array<{ id, qrId, slug, scannedAt, ip, userAgent }>
   qrCodes: [],