fix(audio): remove speed from TTS/cache/MinIO keys; fix presigned URL host rewrite

- Speed is no longer part of Kokoro generation, in-memory cache keys, or
  MinIO object keys — audio is always generated at 1.0 and playback speed
  is applied client-side via audioEl.playbackRate
- presignAudio() now calls rewriteHost() so chapter audio URLs use the
  public MinIO endpoint (same as presignVoiceSample already did)
- docker-compose.yml: rename MINIO_PUBLIC_ENDPOINT → PUBLIC_MINIO_PUBLIC_URL
  for the ui service so SvelteKit's $env/dynamic/public picks it up

ui/src/lib/server/minio.ts

@@ -98,14 +98,12 @@ export async function presignVoiceSample(voice: string): Promise<string> {
 export async function presignAudio(
 	slug: string,
 	n: number,
-	voice?: string,
-	speed?: number
+	voice?: string
 ): Promise<string> {
 	const params = new URLSearchParams();
 	if (voice) params.set('voice', voice);
-	if (speed) params.set('speed', String(speed));
 	const qs = params.toString() ? `?${params.toString()}` : '';
-	log.debug('minio', 'presigning audio', { slug, n, voice, speed });
+	log.debug('minio', 'presigning audio', { slug, n, voice });
 	let res: Response;
 	try {
 		res = await fetch(`${SCRAPER_URL}/api/presign/audio/${slug}/${n}${qs}`);
@@ -126,7 +124,5 @@ export async function presignAudio(
 	}
 	const data = (await res.json()) as { url: string };
 	log.debug('minio', 'presign audio ok', { slug, n });
-	// The scraper now signs audio URLs with the public endpoint directly,
-	// so no host rewrite is needed here.
-	return data.url;
+	return rewriteHost(data.url);
 }