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

docker-compose.yml

@@ -175,7 +175,7 @@ services:
       POCKETBASE_URL: "http://pocketbase:8090"
       POCKETBASE_ADMIN_EMAIL: "${POCKETBASE_ADMIN_EMAIL:-admin@libnovel.local}"
       POCKETBASE_ADMIN_PASSWORD: "${POCKETBASE_ADMIN_PASSWORD:-changeme123}"
-      MINIO_PUBLIC_ENDPOINT: "${MINIO_PUBLIC_ENDPOINT:-http://localhost:9000}"
+      PUBLIC_MINIO_PUBLIC_URL: "${MINIO_PUBLIC_ENDPOINT:-http://localhost:9000}"
     ports:
       - "${UI_PORT:-5252}:3000"
     healthcheck: