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()

ui/src/lib/components/AudioPlayer.svelte

@@ -603,13 +603,17 @@
 				return;
 			}
 
-			// CF AI (batch-only) or already enqueued by presign: keep the traditional
-			// POST → poll → presign flow. For enqueued, we skip the POST and poll.
+			// CF AI voices: use preview/swap strategy.
+			// 1. Fetch a short ~1-2 min preview clip from the first text chunk
+			//    so playback starts immediately — no more waiting behind a spinner.
+			// 2. Meanwhile keep polling the full audio job; when it finishes,
+			//    swap the <audio> src to the full URL preserving currentTime.
 			audioStore.status = 'generating';
+			audioStore.isPreview = false;
 			startProgress();
 
-			// presignResult.enqueued=true means /api/presign/audio already POSTed on our
-			// behalf — skip the duplicate POST and go straight to polling.
+			// Kick off the full audio generation task in the background
+			// (presignResult.enqueued=true means the presign endpoint already did it).
 			if (!presignResult.enqueued) {
 				const res = await fetch(`/api/audio/${slug}/${chapter}`, {
 					method: 'POST',
@@ -618,7 +622,6 @@
 				});
 
 				if (res.status === 402) {
-					// Free daily limit reached — surface upgrade CTA
 					audioStore.status = 'idle';
 					stopProgress();
 					onProRequired?.();
@@ -628,37 +631,96 @@
 				if (!res.ok) throw new Error(`Generation failed: HTTP ${res.status}`);
 
 				if (res.status === 200) {
-					// Already cached — body is { status: 'done' }, no url needed.
+					// Already cached — fast path: presign and play directly.
 					await res.body?.cancel();
 					await finishProgress();
 					const doneUrl = await tryPresign(slug, chapter, voice);
 					if (!doneUrl.ready) throw new Error('Audio generated but presign returned 404');
+					audioStore.isPreview = false;
 					audioStore.audioUrl = doneUrl.url;
 					audioStore.status = 'ready';
 					maybeStartPrefetch();
 					return;
 				}
-				// 202: fall through to polling below.
+				// 202 accepted — fall through: start preview while runner generates
 			}
 
-			// Poll until the runner finishes generating.
-			const final = await pollAudioStatus(slug, chapter, voice);
-			if (final.status === 'failed') {
-				throw new Error(
-					`Generation failed: ${(final as { error?: string }).error ?? 'unknown error'}`
-				);
+			// Fetch the preview clip (first ~1-2 min chunk).
+			// Use an AbortController so we can cancel the background polling if the
+			// user navigates away or stops playback before the full audio is ready.
+			const previewAbort = new AbortController();
+			const qs = new URLSearchParams({ voice });
+			const previewUrl = `/api/audio-preview/${slug}/${chapter}?${qs}`;
+
+			try {
+				const previewRes = await fetch(previewUrl, { signal: previewAbort.signal });
+				if (previewRes.status === 402) {
+					audioStore.status = 'idle';
+					stopProgress();
+					onProRequired?.();
+					return;
+				}
+				if (!previewRes.ok) throw new Error(`Preview failed: HTTP ${previewRes.status}`);
+
+				// The backend responded with the MP3 bytes (or a redirect to MinIO).
+				// Build a blob URL so we can swap it out later without reloading the page.
+				const previewBlob = await previewRes.blob();
+				const previewBlobUrl = URL.createObjectURL(previewBlob);
+
+				audioStore.isPreview = true;
+				audioStore.audioUrl = previewBlobUrl;
+				audioStore.status = 'ready';
+				// Don't restore saved time here — preview is always from 0.
+				// Kick off prefetch of next chapter in the background.
+				maybeStartPrefetch();
+			} catch (previewErr: unknown) {
+				if (previewErr instanceof DOMException && previewErr.name === 'AbortError') return;
+				// Preview failed — fall through to the spinner (old behaviour).
+				// We'll wait for the full audio to finish instead.
+				audioStore.isPreview = false;
 			}
 
-			await finishProgress();
+			// Background: poll for full audio; when done, swap src preserving position.
+			try {
+				const final = await pollAudioStatus(slug, chapter, voice, 2000, previewAbort.signal);
+				if (final.status === 'failed') {
+					throw new Error(
+						`Generation failed: ${(final as { error?: string }).error ?? 'unknown error'}`
+					);
+				}
 
-			// Audio is ready in MinIO — always use a presigned URL for direct playback.
-			const doneUrl = await tryPresign(slug, chapter, voice);
-			if (!doneUrl.ready) throw new Error('Audio generated but presign returned 404');
-			audioStore.audioUrl = doneUrl.url;
-			audioStore.status = 'ready';
-			// Don't restore time for freshly generated audio — position is 0
-			// Immediately start pre-generating the next chapter in background.
-			maybeStartPrefetch();
+				await finishProgress();
+
+				const doneUrl = await tryPresign(slug, chapter, voice);
+				if (!doneUrl.ready) throw new Error('Audio generated but presign returned 404');
+
+				// Swap: save currentTime → update URL → seek to saved position.
+				const savedTime = audioStore.currentTime;
+				const blobUrlToRevoke = audioStore.audioUrl; // capture before overwrite
+				audioStore.isPreview = false;
+				audioStore.audioUrl = doneUrl.url;
+				// If we never started a preview (preview fetch failed), switch to ready now.
+				if (audioStore.status !== 'ready') audioStore.status = 'ready';
+				// The layout $effect will load the new src and auto-play from 0.
+				// We seek back to savedTime after a short delay to let the element
+				// attach the new source before accepting a seek.
+				if (savedTime > 0) {
+					setTimeout(() => {
+						audioStore.seekRequest = savedTime;
+					}, 300);
+				}
+				// Revoke the preview blob URL to free memory.
+				// (We need to wait until the new src is playing; 2 s is safe.)
+				setTimeout(() => {
+					if (blobUrlToRevoke.startsWith('blob:')) {
+						URL.revokeObjectURL(blobUrlToRevoke);
+					}
+				}, 2000);
+				maybeStartPrefetch();
+			} catch (pollErr: unknown) {
+				if (pollErr instanceof DOMException && pollErr.name === 'AbortError') return;
+				throw pollErr;
+			}
 		} catch (e) {
 			stopProgress();
 			audioStore.progress = 0;