Comprehensive architecture documentation including:
- Clean Architecture with Controller-based API (NOT Minimal API)
- Vertical slice architecture pattern
- Exception-based error handling (no Result<T>)
- Feature-driven development approach
- Primary constructor coding standards
- Git commit guidelines
- Swiss QR Bill backward compatibility decisions
Key decisions documented:
- Windows-only targeting (no Linux support needed)
- Support BOTH multipart/form-data AND Base64 JSON
- Separate endpoints for Combined Address (K-Type) legacy support
- Multi-tenancy deferred until after all sync features complete