Two usecases:
- Ships stuck somewhere need a way to send them back to a location with fuel. This not only includes planets but also the CX.
- Transferring STL ships from one system to another.
The towing service could either be just a button in the fleet menu or a full blown towing station built on planets so ships can only be towed to places with prebuilt infrastructure. In the latter case the station will charge a fee based on the distance. Cap the fee to 5k for starter ships.