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>

src/AdminPage.tsx

@@ -564,6 +564,12 @@ export interface AdminStats {
     }>
     funnel?: { signups: number; firstVisit: number; firstCompletion: number }
   }
+  episodePlays?: Array<{
+    title: string
+    total: number
+    byDay: Record<string, number>
+    last30Days: Array<{ day: string; plays: number }>
+  }>
 }
 
 interface AdminAsset {