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/search/+server.ts

@@ -0,0 +1,34 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { log } from '$lib/server/logger';
+import { backendFetch } from '$lib/server/scraper';
+
+/**
+ * GET /api/search?q=<query>
+ * Proxies to the Go backend's /api/search endpoint.
+ * Returns: { results, local_count, remote_count }
+ *
+ * Response shape mirrors SearchResponse in the iOS APIClient.
+ */
+export const GET: RequestHandler = async ({ url }) => {
+	const q = url.searchParams.get('q') ?? '';
+
+	if (q.trim().length < 2) {
+		return json({ results: [], local_count: 0, remote_count: 0 });
+	}
+
+	const apiURL = `/api/search?q=${encodeURIComponent(q.trim())}`;
+	try {
+		const res = await backendFetch(apiURL);
+		if (!res.ok) {
+			log.error('api/search', 'backend returned error', { status: res.status, q });
+			error(502, `Search failed: ${res.status}`);
+		}
+		const data = await res.json();
+		return json(data);
+	} catch (e) {
+		if (e instanceof Error && 'status' in e) throw e;
+		log.error('api/search', 'network error', { q, err: String(e) });
+		error(502, 'Could not reach backend');
+	}
+};