chore: migrate to v3, Doppler secrets, clean up legacy code

- Remove all pre-v3 code: scraper, ui-v2, backend v1, ios v1+v2, legacy CI workflows
- Flatten v3/ contents to repo root
- Add Doppler secrets management (project=libnovel, config=prd)
- Add justfile with doppler run wrappers for all docker compose commands
- Strip hardcoded env fallbacks from docker-compose.yml
- Add minimal README.md
- Clean up .gitignore

ui/.env.example

@@ -1,9 +1,9 @@
 # libnovel UI — environment variables
 # Copy to .env and adjust; do NOT commit with real secrets.
 
-# Public URL of the scraper API (used by SvelteKit server-side load functions)
+# Internal URL of the backend API (used by SvelteKit server-side load functions)
 # In docker-compose this is the internal service name
-SCRAPER_API_URL=http://localhost:8080
+BACKEND_API_URL=http://localhost:8080
 
 # Public URL of PocketBase (used by SvelteKit server-side load functions)
 POCKETBASE_URL=http://localhost:8090