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>

CLAUDE.md

@@ -130,6 +130,7 @@ cd backend && dotnet build
 - Controllers use attribute routing: `[Route("api/[controller]")]`
 - Health check: `GET /api/health` — returns `{ status, database, environment }`
 - Health ping: `GET /api/health/ping` — returns `{ status: "pong" }`
+- Upload: `POST /api/upload` — multipart form, fields: `file` (required), `path` (optional folder prefix)
 
 ## Frontend Conventions
 
@@ -168,6 +169,20 @@ Host=postgres;Port=5432;Database={db};Username={user};Password={pass}
 Configured via env var `ConnectionStrings__Default` in docker-compose files.
 Locally via `appsettings.Local.json` with SSH tunnel (`localhost:5433`).
 
+## CDN & Image Storage
+
+- **Storage:** Cloudflare R2 (S3-compatible)
+- **CDN domain:** https://cdn.goodbrick.com.ua
+- **Bucket:** goodbrick
+- **Frontend:** `NEXT_PUBLIC_CDN_URL` env var, `cdnUrl()` helper in `src/lib/cdn.ts`
+- **Backend:** `R2__*` env vars, `R2StorageService` for uploads
+- **Upload endpoint:** `POST /api/upload` — multipart/form-data, max 10MB, images only (jpeg/png/webp/avif)
+
+### Folder Convention
+- `products/<slug>/main.jpg` — product photos
+- `catalog/<slug>/cover.jpg` — collection covers
+- `site/<name>.jpg` — site-wide images (hero, about, etc.)
+
 ## Key Rules
 
 - Never commit `.env` files or secrets — credentials are in docker-compose env vars on server