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/.env.example

@@ -0,0 +1,20 @@
+# libnovel UI — environment variables
+# Copy to .env and adjust; do NOT commit with real secrets.
+
+# Internal URL of the backend API (used by SvelteKit server-side load functions)
+# In docker-compose this is the internal service name
+BACKEND_API_URL=http://localhost:8080
+
+# Public URL of PocketBase (used by SvelteKit server-side load functions)
+POCKETBASE_URL=http://localhost:8090
+
+# PocketBase admin credentials (server-side only, never exposed to browser)
+POCKETBASE_ADMIN_EMAIL=admin@libnovel.local
+POCKETBASE_ADMIN_PASSWORD=changeme123
+
+# Public-facing MinIO URL (used to rewrite presigned URLs for the browser)
+# In dev this is localhost; in prod set to your MinIO public domain
+PUBLIC_MINIO_PUBLIC_URL=http://localhost:9000
+
+# Secret used to sign auth tokens stored in cookies (generate with: openssl rand -hex 32)
+AUTH_SECRET=change_this_to_a_long_random_secret