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/routes/api/presign/audio/+server.ts

@@ -7,6 +7,7 @@ import { log } from '$lib/server/logger';
  * GET /api/presign/audio?slug=...&n=...&voice=...&speed=...
  * Returns a presigned MinIO URL for the audio file so the browser
  * can stream it directly without going through the server.
+ * Returns 404 when the audio has not been generated yet.
  */
 export const GET: RequestHandler = async ({ url }) => {
 	const slug = url.searchParams.get('slug');
@@ -22,6 +23,10 @@ export const GET: RequestHandler = async ({ url }) => {
 		const presignedUrl = await presignAudio(slug, n, voice, speed);
 		return json({ url: presignedUrl });
 	} catch (e) {
+		const status = (e as { status?: number }).status;
+		if (status === 404) {
+			error(404, 'Audio not found');
+		}
 		log.error('presign', 'presign audio failed', { slug, n, err: String(e) });
 		error(500, `Could not get presigned URL: ${e}`);
 	}