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/data.js

@@ -28,6 +28,7 @@ import {
   UPLOADS_DIR,
   UPLOADS_META_FILE,
   DOWNLOAD_COUNTS_FILE,
+  EPISODE_PLAYS_FILE,
   EMPTY_HIT_STATS,
   EMPTY_VISITOR_STATS,
   DEFAULT_REPLY_TEMPLATES,
@@ -675,6 +676,40 @@ export function incrementDownloadCount(resourceKey) {
   queueDownloadCountsWrite()
 }
 
+// ── Episode play counts ────────────────────────────────────────────────────
+
+export function queueEpisodePlaysWrite() {
+  state.episodePlaysWritePromise = state.episodePlaysWritePromise
+    .then(async () => {
+      await mkdir(DATA_DIR, { recursive: true })
+      await writeFile(EPISODE_PLAYS_FILE, JSON.stringify(state.episodePlays, null, 2), 'utf8')
+    })
+    .catch(err => {
+      console.error('[episode-plays] failed to write:', err)
+    })
+}
+
+export function loadEpisodePlaysFromDisk() {
+  return readFile(EPISODE_PLAYS_FILE, 'utf8')
+    .then(raw => {
+      const parsed = JSON.parse(raw)
+      state.episodePlays = (parsed && typeof parsed === 'object' && !Array.isArray(parsed)) ? parsed : {}
+    })
+    .catch(() => {
+      state.episodePlays = {}
+    })
+}
+
+export function recordEpisodePlay(title) {
+  const today = new Date().toISOString().slice(0, 10)
+  if (!state.episodePlays[title]) {
+    state.episodePlays[title] = { total: 0, byDay: {} }
+  }
+  state.episodePlays[title].total += 1
+  state.episodePlays[title].byDay[today] = (state.episodePlays[title].byDay[today] ?? 0) + 1
+  queueEpisodePlaysWrite()
+}
+
 // ── Uploads ────────────────────────────────────────────────────────────────
 
 export async function readUploadsMetadata() {