Refactor domain structure to business layer

- Renamed `domain/` to `business/` to better reflect the organization of code by business logic.
- Updated all relevant file paths and references throughout the project to align with the new structure.
- Ensured that all models and services are now located under the `business/` directory, maintaining existing functionality while improving clarity.

docs/refactor/folder-structure/README.md

@@ -21,7 +21,7 @@ backend/igny8_core/
 ```
 backend/igny8_core/
 ├── core/            # Core models (Account, User, Site, Sector)
-├── domain/          # Domain-specific code
+├── business/          # Domain-specific code
 │   ├── content/     # Content domain
 │   ├── planning/    # Planning domain
 │   ├── linking/     # Linking domain
@@ -52,8 +52,8 @@ backend/igny8_core/
 
 ## File Organization Rules
 
-- **Models**: `domain/{domain}/models.py`
-- **Services**: `domain/{domain}/services/`
+- **Models**: `business/{business}/models.py`
+- **Services**: `business/{business}/services/`
 - **Serializers**: `modules/{module}/serializers.py`
 - **ViewSets**: `modules/{module}/views.py`
 - **URLs**: `modules/{module}/urls.py`