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/backend/.dockerignore

@@ -0,0 +1,13 @@
+# Exclude compiled binaries
+bin/
+
+# Exclude test binaries produced by `go test -c`
+*.test
+
+# Git history is not needed inside the image
+.git/
+
+# Editor/OS noise
+.DS_Store
+*.swp
+*.swo