feat(clean-release): complete and verify backend test suite (33 passing tests)

- Relocated and standardized tests for clean_release subsystem into __tests__ sub-packages.
- Implemented missing unit tests for preparation_service, audit_service, and stages.
- Enhanced API contract tests for candidate preparation and compliance reporting.
- Updated 023-clean-repo-enterprise coverage matrix with final verification results.
- Fixed relative import issues and model validation mismatches during test migration.

specs/023-clean-repo-enterprise/tests/coverage.md

@@ -7,7 +7,10 @@
 | `clean_release.report_builder` | `report_builder.py` | CRITICAL | ✅ Yes | 1/1 | 3/3 | 1/1 |
 | `clean_release.manifest_builder` | `manifest_builder.py` | STANDARD | ✅ Yes | N/A | N/A | N/A |
 | `clean_release.source_isolation` | `source_isolation.py` | STANDARD | ✅ Yes | N/A | N/A | N/A |
-| `api.routes.clean_release` | `clean_release.py` | STANDARD | ✅ Yes | N/A | N/A | N/A |
+| `clean_release.preparation_service` | `preparation_service.py` | STANDARD | ✅ Yes | 1/1 | 2/2 | 1/1 |
+| `clean_release.audit_service` | `audit_service.py` | STANDARD | ✅ Yes | N/A | N/A | 1/1 |
+| `clean_release.stages` | `stages.py` | STANDARD | ✅ Yes | N/A | 3/3 | N/A |
+| `api.routes.clean_release` | `clean_release.py` | STANDARD | ✅ Yes | 1/1 | 2/2 | 1/1 |
 | `api.routes.tasks.get_task_logs` | `tasks.py` | CRITICAL | ✅ Yes | 1/1 | 3/3 | 1/1 |
 | `models.clean_release` | `clean_release.py` | CRITICAL | ✅ Yes | 1/1 | 3/3 | 1/1 |
 | `frontend.assistant_chat.integration` | `assistant_chat.integration.test.js` | CRITICAL | ✅ Yes | 1/1 | 3/3 | 1/1 |