Statistics for Yap volunteers

Would be very useful for us to have statistics available for the Volunteers in Yap.

Specifically, a count of volunteers per service body and optional count of male versus female for each.

Look for existing volunteer data models and service body relationships in the codebase, likely in the backend PHP files. Start by searching for 'volunteer' and 'service body' to understand the data structure. Then, determine where statistics are currently generated or displayed, and add new counts for volunteers per service body and gender breakdown. Verify the changes by checking if there are existing tests for statistics or data reporting.

Written by the indexing model from the issue text.