add yard distances management to rule engine

- Introduced new yard distances resource with CRUD operations.
- Created migration for yard distances table with necessary constraints.
- Implemented service and repository for yard distances handling.
- Added controller for API endpoints to manage yard distances.
- Updated rule engine configuration to include yard distances.
- Enhanced rule engine resource page to support yard distance selection.
- Updated contracts and train builder pages to handle new yard distance logic.
- Added error handling utility for better error message extraction.

apps/edr-freight-web/backoffice/src/components/trainBuilder/BuildTrainModal.tsx

@@ -46,10 +46,18 @@ export default function BuildTrainModal({ opened, onClose, onBuilt }: BuildTrain
   // Admin-managed run list (dropdown settings); numbers already on a train
   // come back disabled so they cannot be picked twice.
   const importNumbers = useImportTrainNumberOptions();
-  // Only serviceable locomotives standing in the selected yard can be coupled.
+  // Only serviceable locomotives standing in the selected yard, and not already
+  // coupled to another built train, can be picked. A new train owns none yet, so
+  // no train to keep-exclude.
   const locomotivesQuery = useQuery(
     api.locomotives.listFiltered.queryOptions({
-      input: { filters: { status: "AVAILABLE", currentYardId: yardId } },
+      input: {
+        filters: {
+          status: "AVAILABLE",
+          currentYardId: yardId,
+          excludeCoupled: true,
+        },
+      },
       enabled: Boolean(yardId),
     }),
   );