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/components.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "https://shadcn-svelte.com/schema.json",
+  "style": "default",
+  "tailwind": {
+    "config": "",
+    "css": "src/app.css",
+    "baseColor": "zinc",
+    "cssVariables": false
+  },
+  "aliases": {
+    "components": "$lib/components",
+    "utils": "$lib/utils",
+    "ui": "$lib/components/ui",
+    "hooks": "$lib/hooks"
+  },
+  "typescript": true
+}