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/public/robots.txt

@@ -0,0 +1,5 @@
+User-agent: *
+Allow: /
+Allow: /_next/image
+
+Sitemap: https://new.goodbrick.com.ua/sitemap.xml