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/svelte.config.js

@@ -0,0 +1,10 @@
+import adapter from '@sveltejs/adapter-node';
+
+/** @type {import('@sveltejs/kit').Config} */
+const config = {
+	kit: {
+		adapter: adapter()
+	}
+};
+
+export default config;