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>

deploy/docker-compose.local.yml

@@ -7,3 +7,8 @@ services:
     environment:
       - ASPNETCORE_ENVIRONMENT=Development
       - ConnectionStrings__Default=Host=host.docker.internal;Port=5433;Database=dev_db;Username=dev_user;Password=dev_pass_vB6nM3qP8yW2rT9k
+      - R2__AccountId=${R2_ACCOUNT_ID}
+      - R2__AccessKeyId=${R2_ACCESS_KEY_ID}
+      - R2__SecretAccessKey=${R2_SECRET_ACCESS_KEY}
+      - R2__BucketName=${R2_BUCKET_NAME:-goodbrick}
+      - R2__PublicUrl=https://cdn.goodbrick.com.ua