fix(lint): restore noqa directives and register custom codes with ruff

Reverts the noqa: removal — the architecture validators (SVC-006,
SEC-034, MOD-004, API-007) use these to skip known-safe violations.
Added ruff lint.external config so ruff treats them as valid codes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/monitoring/services/background_tasks_service.py

@@ -44,7 +44,7 @@ class BackgroundTasksService:
 
     def get_running_test_runs(self, db: Session) -> list[TestRun]:
         """Get currently running test runs"""
-        # SVC-005 - Platform-level, TestRuns not store-scoped
+        # noqa: SVC-005 - Platform-level, TestRuns not store-scoped
         return db.query(TestRun).filter(TestRun.status == "running").all()
 
     def get_import_stats(self, db: Session) -> dict: