Refactored EKÁER declaration logic to allow one EkaerHistory to reference multiple source documents via a new EkaerHistoryMapping junction table. Removed ForeignKey from EkaerHistory and updated all usages to use the Mappings collection. Updated service, controller, SignalR, and client interfaces to operate by EkaerHistoryId. Adjusted grid UI to display all mapped source IDs. Added EkaerHistoryMapping entity and updated documentation, constants, and ToonDescription attributes accordingly.
