Remove center bias by buffering city by radius

See Copenhagen - there are almost no gaps found within 2500m of the city boundary:

Implementing this for the batch export is nontrivial since pbfs would need to be recreated with a buffer.

Start by locating the batch export entry point and the code that creates the PBFS used for city-boundary analysis. Determine how to apply a radius buffer before recreating those PBFS, then validate the result against the Copenhagen case and the 2500m boundary described in the issue.

Written by the indexing model from the issue text.