fix(external-system-gateway): resolve ExternalSystemGateway-011 — name-keyed repository lookups replace fetch-all-then-filter on the call hot path

src/ScadaLink.ExternalSystemGateway/DatabaseGateway.cs

@@ -170,8 +170,10 @@ public class DatabaseGateway : IDatabaseGateway
         string connectionName,
         CancellationToken cancellationToken)
     {
-        var connections = await _repository.GetAllDatabaseConnectionsAsync(cancellationToken);
-        return connections.FirstOrDefault(c =>
-            c.Name.Equals(connectionName, StringComparison.OrdinalIgnoreCase));
+        // ExternalSystemGateway-011: name-keyed repository lookup instead of
+        // fetch-all-then-filter — connection definitions are resolved on every
+        // cached write / connection request, so the repository performs an indexed
+        // query rather than loading every connection into memory.
+        return await _repository.GetDatabaseConnectionByNameAsync(connectionName, cancellationToken);
     }
 }