feat(player): CF AI preview/swap + fix PB token expiry + local build time

- Backend: add GET /api/audio-preview/{slug}/{n} — generates first ~1800-char
  chunk via CF AI so playback starts immediately; full chapter cached in MinIO
- Frontend: replace CF AI spinner with preview blob URL + background swap to
  full presigned URL when runner finishes, preserving currentTime
- AudioPlayer: isPreview state + 'preview' badge in mini-bar during swap
- pocketbase.ts: fix 403 on stale token — reduce TTL to 50 min + retry once
  on 401/403 with forced re-auth (was cached 12 h, PB tokens expire in 1 h)
- Footer build time now rendered in user's local timezone via toLocaleString()

backend/internal/backend/handlers.go

@@ -904,6 +904,115 @@ func (s *Server) handleAudioStream(w http.ResponseWriter, r *http.Request) {
 	// on its next poll as soon as the MinIO object is present.
 }
 
+// handleAudioPreview handles GET /api/audio-preview/{slug}/{n}.
+//
+// CF AI voices are batch-only and can take 1-2+ minutes to generate a full
+// chapter. This endpoint generates only the FIRST chunk of text (~1 800 chars,
+// roughly 1-2 minutes of audio) so the client can start playing immediately
+// while the full audio is generated in the background by the runner.
+//
+// Fast path: if a preview object already exists in MinIO, redirects to its
+// presigned URL (no regeneration).
+//
+// Slow path: generates the first chunk via CF AI, streams the MP3 bytes to the
+// client, and simultaneously uploads to MinIO under a "_preview" key so future
+// requests hit the fast path.
+//
+// Only CF AI voices are expected here. Calling this with a Kokoro/PocketTTS
+// voice falls back to the normal audio-stream endpoint behaviour.
+//
+// Query params:
+//
+//	voice  (required — must be a cfai: voice)
+func (s *Server) handleAudioPreview(w http.ResponseWriter, r *http.Request) {
+	slug := r.PathValue("slug")
+	n, err := strconv.Atoi(r.PathValue("n"))
+	if err != nil || n < 1 {
+		jsonError(w, http.StatusBadRequest, "invalid chapter")
+		return
+	}
+
+	voice := r.URL.Query().Get("voice")
+	if voice == "" {
+		voice = s.cfg.DefaultVoice
+	}
+
+	if s.deps.CFAI == nil {
+		jsonError(w, http.StatusServiceUnavailable, "cloudflare AI TTS not configured")
+		return
+	}
+
+	// Preview key: same as normal key with a "_preview" suffix before the extension.
+	// e.g. slug/1/cfai:luna_preview.mp3
+	previewKey := s.deps.AudioStore.AudioObjectKeyExt(slug, n, voice+"_preview", "mp3")
+
+	// ── Fast path: preview already in MinIO ──────────────────────────────────
+	if s.deps.AudioStore.AudioExists(r.Context(), previewKey) {
+		presignURL, err := s.deps.PresignStore.PresignAudio(r.Context(), previewKey, 1*time.Hour)
+		if err != nil {
+			s.deps.Log.Error("handleAudioPreview: PresignAudio failed", "slug", slug, "n", n, "err", err)
+			jsonError(w, http.StatusInternalServerError, "presign failed")
+			return
+		}
+		http.Redirect(w, r, presignURL, http.StatusFound)
+		return
+	}
+
+	// ── Slow path: generate first chunk + stream + save ──────────────────────
+
+	// Read the chapter text.
+	raw, err := s.deps.BookReader.ReadChapter(r.Context(), slug, n)
+	if err != nil {
+		s.deps.Log.Error("handleAudioPreview: ReadChapter failed", "slug", slug, "n", n, "err", err)
+		jsonError(w, http.StatusNotFound, "chapter not found")
+		return
+	}
+	text := stripMarkdown(raw)
+	if text == "" {
+		jsonError(w, http.StatusUnprocessableEntity, "chapter text is empty")
+		return
+	}
+
+	// Take only the first ~1 800 characters — one CF AI chunk, roughly 1-2 min.
+	const previewChars = 1800
+	firstChunk := text
+	if len([]rune(text)) > previewChars {
+		runes := []rune(text)
+		firstChunk = string(runes[:previewChars])
+		// Walk back to last sentence boundary (. ! ?) to avoid a mid-word cut.
+		for i := previewChars - 1; i > previewChars/2; i-- {
+			r := runes[i]
+			if r == '.' || r == '!' || r == '?' || r == '\n' {
+				firstChunk = string(runes[:i+1])
+				break
+			}
+		}
+	}
+
+	// Generate the preview chunk via CF AI.
+	mp3, err := s.deps.CFAI.GenerateAudio(r.Context(), firstChunk, voice)
+	if err != nil {
+		s.deps.Log.Error("handleAudioPreview: GenerateAudio failed", "slug", slug, "n", n, "voice", voice, "err", err)
+		jsonError(w, http.StatusInternalServerError, "tts generation failed")
+		return
+	}
+
+	// Upload to MinIO in the background so the next request hits the fast path.
+	go func() {
+		if uploadErr := s.deps.AudioStore.PutAudio(
+			context.Background(), previewKey, mp3,
+		); uploadErr != nil {
+			s.deps.Log.Error("handleAudioPreview: MinIO upload failed", "key", previewKey, "err", uploadErr)
+		}
+	}()
+
+	w.Header().Set("Content-Type", "audio/mpeg")
+	w.Header().Set("Content-Length", strconv.Itoa(len(mp3)))
+	w.Header().Set("Cache-Control", "no-store")
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write(mp3)
+}
+
 // ── Translation ────────────────────────────────────────────────────────────────
 
 // supportedTranslationLangs is the set of target locales the backend accepts.

backend/internal/backend/server.go

@@ -180,6 +180,9 @@ func (s *Server) ListenAndServe(ctx context.Context) error {
 	// Streaming audio: serves from MinIO if cached, else streams live TTS
 	// while simultaneously uploading to MinIO for future requests.
 	mux.HandleFunc("GET /api/audio-stream/{slug}/{n}", s.handleAudioStream)
+	// CF AI preview: generates only the first ~1 800-char chunk so the client
+	// can start playing immediately while the full audio is generated by the runner.
+	mux.HandleFunc("GET /api/audio-preview/{slug}/{n}", s.handleAudioPreview)
 
 	// Translation task creation (backend creates task; runner executes via LibreTranslate)
 	mux.HandleFunc("POST /api/translation/{slug}/{n}", s.handleTranslationGenerate)