feat(v3): add v3 stack — backend rewrite, renamed env vars, docs

- New Go backend binary (backend + runner) replacing old scraper/
- Rename SCRAPER_API_URL → BACKEND_API_URL in UI env and docker-compose
- Rename scraperFetch → backendFetch across all 19 UI server files
- Remove SCRAPER_PROXY env var and proxy transport from browser.Config
- Add Meilisearch, Valkey, Caddy to docker-compose
- Add docs/: api-endpoints.md, request-flow.mermaid.md, data-flow.mermaid.md

v3/ui/src/routes/api/presign/voice-sample/+server.ts

@@ -0,0 +1,40 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { presignVoiceSample } from '$lib/server/minio';
+import * as cache from '$lib/server/presignCache';
+
+/**
+ * GET /api/presign/voice-sample?voice=af_bella
+ * Returns a presigned URL for the voice sample audio file.
+ * Returns 404 if the sample has not been generated yet.
+ *
+ * Results are cached in-process for 50 minutes to avoid a backend + MinIO
+ * round-trip on every voice-selection preview play.
+ */
+export const GET: RequestHandler = async ({ url }) => {
+	const voice = url.searchParams.get('voice');
+	if (!voice) {
+		error(400, 'Missing voice parameter');
+	}
+
+	const cacheKey = cache.sampleKey(voice);
+
+	// Fast path: return cached URL if still valid.
+	const cached = await cache.get(cacheKey);
+	if (cached) {
+		return json({ url: cached });
+	}
+
+	// Slow path: call backend → MinIO presign.
+	try {
+		const presignedUrl = await presignVoiceSample(voice);
+		await cache.set(cacheKey, presignedUrl);
+		return json({ url: presignedUrl });
+	} catch (e) {
+		const status = (e as { status?: number }).status;
+		if (status === 404) {
+			error(404, 'Voice sample not found');
+		}
+		error(502, `Failed to presign voice sample: ${e}`);
+	}
+};