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/vite.config.ts

@@ -0,0 +1,18 @@
+import { sveltekit } from '@sveltejs/kit/vite';
+import tailwindcss from '@tailwindcss/vite';
+import { defineConfig } from 'vite';
+
+export default defineConfig({
+	plugins: [tailwindcss(), sveltekit()],
+	ssr: {
+		// Force these packages to be bundled into the server output rather than
+		// treated as external requires. The production Docker image has no
+		// node_modules, so anything used in server-side code must be inlined.
+		noExternal: ['marked'],
+		// cropperjs is DOM-only (used inside $effect); exclude from SSR bundle.
+		external: ['cropperjs']
+	},
+	optimizeDeps: {
+		include: ['cropperjs']
+	}
+});