Baked-in Slippy Tile server

It might be nice to allow for direct serving of tiles from the server running Tilehuria-flask.
- The tiles are already downloaded onto the server. It would be handy to have them instantly available for a mapathon.
- This would be particularly useful if someone is planning a mapathon and preps a POSM-style server beforehand (or even in a high-bandwidth country prior to traveling to a low-bandwidth place). In some cases serving tiles on a LAN might be preferable to distributing MBTiles on removable media.

Start at the Tilehuria-flask server entry point and inspect how downloaded tiles are stored. Compare the requested LAN serving path with the existing MBTiles and removable-media use cases. Done means the server can directly serve prepared tiles for a mapathon without requiring separate distribution.

Written by the indexing model from the issue text.