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/package.json

@@ -0,0 +1,37 @@
+{
+	"name": "ui",
+	"private": true,
+	"version": "0.0.1",
+	"type": "module",
+	"scripts": {
+		"dev": "vite dev",
+		"build": "vite build",
+		"preview": "vite preview",
+		"prepare": "svelte-kit sync || echo ''",
+		"check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
+		"check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch"
+	},
+	"devDependencies": {
+		"@sveltejs/adapter-auto": "^7.0.0",
+		"@sveltejs/adapter-node": "^5.5.4",
+		"@sveltejs/kit": "^2.50.2",
+		"@sveltejs/vite-plugin-svelte": "^6.2.4",
+		"@tailwindcss/vite": "^4.2.1",
+		"@types/node": "^25.3.3",
+		"clsx": "^2.1.1",
+		"svelte": "^5.51.0",
+		"svelte-check": "^4.4.2",
+		"tailwind-merge": "^3.5.0",
+		"tailwindcss": "^4.2.1",
+		"typescript": "^5.9.3",
+		"vite": "^7.3.1"
+	},
+	"dependencies": {
+		"@aws-sdk/client-s3": "^3.1005.0",
+		"@aws-sdk/s3-request-presigner": "^3.1005.0",
+		"cropperjs": "^1.6.2",
+		"ioredis": "^5.3.2",
+		"marked": "^17.0.3",
+		"pocketbase": "^0.26.8"
+	}
+}