Add R2 image storage, upload endpoint, and CDN support

- Backend: R2StorageService, upload controller (POST /api/upload)
- Frontend: CDN url helper, NEXT_PUBLIC_CDN_URL build arg
- Deploy: pass R2 secrets from Woodpecker CI to containers via .env
- Docs: update CLAUDE.md with CDN and upload conventions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/.env.example

@@ -3,3 +3,6 @@ INTERNAL_API_URL=http://gb-prod-backend:5000
 
 # URL бэкенда для проксирования /api/* в локальной разработке (не задавать на сервере!)
 # LOCAL_API_URL=http://localhost:5000
+
+# CDN URL для изображений (Cloudflare R2)
+NEXT_PUBLIC_CDN_URL=https://cdn.goodbrick.com.ua