Catchment report

For a possibly large GJ input representing something like a new scheme built somewhere, calculate

- [ ] POIs within X minutes by some mode
- [x] Intersect some polygon buffer with polygon areas and sum up census/demographic data
- [ ] Output everything visually/interactively, but also as a big downloadable report (specifically what're all the schools close by?)
- [ ] Calculate mesh density specifically

The issue describes a catchment report for a GJ input, involving POI proximity, polygon intersection with census data, visual/interactive output, downloadable reports, and mesh density calculation. Start by examining the codebase for existing data processing and visualization modules, particularly around geographic data handling and report generation. Look for entry points that handle GJ inputs and existing tools for POI analysis or demographic data summation. Determine what 'big downloadable report' entails and how mesh density is currently calculated, if at all.

Written by the indexing model from the issue text.