fix(audio): return 404 from presign when audio object not yet uploaded

handlePresignAudio now checks AudioExists before presigning; previously it
would generate a valid-looking signed URL for a non-existent object, causing
the browser to get 404 from MinIO directly.

- Add AudioExists to Store interface and HybridStore
- Guard handlePresignAudio with AudioExists check, return 404 if missing
- Propagate 404 through presignAudio() in minio.ts (typed error.status=404)
- SvelteKit presign route forwards 404 to the browser instead of 500

ui/src/lib/server/minio.ts

@@ -64,6 +64,7 @@ export async function presignChapter(slug: string, n: number, rewrite = false):
 /**
  * Returns a presigned URL for an audio file.
  * URL is valid for ~1 hour. The URL is returned to the browser for direct streaming.
+ * Throws with { status: 404 } when the audio object has not been generated yet.
  */
 export async function presignAudio(
 	slug: string,
@@ -83,6 +84,12 @@ export async function presignAudio(
 		log.error('minio', 'presign audio network error', { slug, n, err: String(e) });
 		throw new Error(`presign audio ${slug}/${n}: network error`);
 	}
+	if (res.status === 404) {
+		// Audio hasn't been generated / uploaded yet — caller should surface this as 404.
+		const err = new Error(`presign audio ${slug}/${n}: not found`) as Error & { status: number };
+		err.status = 404;
+		throw err;
+	}
 	if (!res.ok) {
 		const body = await res.text().catch(() => '');
 		log.error('minio', 'presign audio failed', { slug, n, status: res.status, body });