Refactor ban management with domain models and mappers

- Add ban domain model for core business logic separation
- Implement mapper pattern for DTO/domain conversions
- Update ban service with new domain-driven approach
- Refactor router endpoints to use new architecture
- Add comprehensive mapper tests
- Update documentation with architecture changes

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/tests/test_mappers/__init__.py

@@ -0,0 +1 @@
+"""Tests for response mappers."""