feat: TTS streaming — Kokoro/PocketTTS audio starts immediately

Previously all voices waited for full TTS generation before first byte
reached the browser (POST → poll → presign flow). For long chapters this
meant 2–5 minutes of silence.

New flow for Kokoro and PocketTTS:
- tryPresign fast path unchanged (audio in MinIO → seekable presigned URL)
- On cache miss: set audioEl.src to /api/audio-stream/{slug}/{n} and mark
  status=ready immediately — audio starts playing within seconds
- Backend streams bytes to browser while concurrently uploading to MinIO;
  subsequent plays use the fast path

New SvelteKit route: GET /api/audio-stream/[slug]/[n]
- Proxies backend handleAudioStream (already implemented in Go)
- Same 3 chapters/day free paywall as POST route
- HEAD handler for paywall pre-check (no side effects, no counter increment)
  so AudioPlayer can surface upgrade CTA before pointing <audio> at URL

CF AI voices keep the old POST+poll flow (batch-only API, no real streaming
benefit, preserves the generating progress bar UX).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/lib/components/AudioPlayer.svelte

@@ -567,7 +567,30 @@
 				return;
 			}
 
-			// Slow path: trigger Kokoro generation (non-blocking POST), then poll.
+			// Slow path: audio not yet in MinIO.
+			//
+			// For Kokoro / PocketTTS when presign has NOT already enqueued the runner:
+			// use the streaming endpoint — audio starts playing within seconds while
+			// generation runs and MinIO is populated concurrently.
+			// Skip when enqueued=true to avoid double-generation with the async runner.
+			if (!voice.startsWith('cfai:') && !presignResult.enqueued) {
+				const qs = new URLSearchParams({ voice, format: 'mp3' });
+				const streamUrl = `/api/audio-stream/${slug}/${chapter}?${qs}`;
+				// HEAD probe: check paywall without triggering generation.
+				const headRes = await fetch(streamUrl, { method: 'HEAD' }).catch(() => null);
+				if (headRes?.status === 402) {
+					audioStore.status = 'idle';
+					onProRequired?.();
+					return;
+				}
+				audioStore.audioUrl = streamUrl;
+				audioStore.status = 'ready';
+				maybeStartPrefetch();
+				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.
 			audioStore.status = 'generating';
 			startProgress();