Describe the bug

• See Replace usage of Nominatim with pg-nearest-city tasking-manager#6905
• When we are getting the nearest city at a country border, the border is not respected.
• Instead, just literally the nearest city is used.

Expected behavior

• Country borders should be respected.
• To achieve this we should also load a dataset of country border polygons.
• The generated voronois should be clipped to country borders, to ensure the 'city' polygons do not cross them.

This is an interesting one!
@emirfabio
@stephanGarland