refactor: sweep the platform-and-sign-in epic

Split load_config into per-section readers, use contextlib.suppress and a single condition where ruff asked, remove stale noqa comments, add the SIM and RUF100 rules, share the sign_in test helper, align AD-6 with the code and record the remaining deferred findings.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N3t4WL7bd9bdHHNjEsxzqC

portal/config/health.py

@@ -17,7 +17,7 @@ def healthz(request):
         with connection.cursor() as cursor:
             cursor.execute("SELECT 1")
             cursor.fetchone()
-    except Exception as exc:  # noqa: BLE001 - any failure means unavailable
+    except Exception as exc:  # any failure means unavailable
         logger.error("health check failed", extra={"error_type": type(exc).__name__})
         return JsonResponse({"status": "unavailable"}, status=503)
     return JsonResponse({"status": "ok"})