chore: update API routers, validation, and docs for module system

- app/api/v1/admin/__init__.py: Updated router imports
- app/api/v1/vendor/__init__.py: Updated router imports
- app/exceptions/code_quality.py: Added module exception imports
- scripts/validate_architecture.py: Added module validation rules
- .architecture-rules/_main.yaml: Include module.yaml rules
- docs/proposals/module-migration-plan.md: Updated migration status

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/exceptions/code_quality.py

@@ -4,6 +4,9 @@ Code Quality Domain Exceptions
 
 These exceptions are raised by the code quality service layer
 and converted to HTTP responses by the global exception handler.
+
+Note: These exceptions are also re-exported from app.modules.dev_tools.exceptions
+for module-based access.
 """
 
 from app.exceptions.base import (