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.

backend/src/api/routes/tasks.py

@@ -182,6 +182,16 @@ async def get_task(
 # @POST: Returns a list of log entries or raises 404.
 # @RETURN: List[LogEntry] - List of log entries.
 # @TIER: CRITICAL
+# @TEST_CONTRACT get_task_logs_api ->
+# {
+#   required_params: {task_id: str},
+#   optional_params: {level: str, source: str, search: str},
+#   invariants: ["returns 404 for non-existent task", "applies filters correctly"]
+# }
+# @TEST_FIXTURE valid_task_logs_request -> {"task_id": "test_1", "level": "INFO"}
+# @TEST_EDGE task_not_found -> raises 404
+# @TEST_EDGE invalid_limit -> Query(limit=0) returns 422
+# @TEST_INVARIANT response_purity -> verifies: [valid_task_logs_request]
 # @TEST_CONTRACT: TaskLogQueryInput -> List[LogEntry]
 # @TEST_SCENARIO: existing_task_logs_filtered -> Returns filtered logs by level/source/search with pagination.
 # @TEST_FIXTURE: valid_task_with_mixed_logs -> backend/tests/fixtures/task_logs/valid_task_with_mixed_logs.json