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/src/lib/cdn.ts

@@ -0,0 +1,6 @@
+const CDN_BASE_URL =
+  process.env.NEXT_PUBLIC_CDN_URL || "https://cdn.goodbrick.com.ua";
+
+export function cdnUrl(path: string): string {
+  return `${CDN_BASE_URL.replace(/\/+$/, "")}/${path.replace(/^\/+/, "")}`;
+}