Investigate how measurements are logged (mean vs. median)

Note from Elsa's logbook:

> 23-11-2018: Målingerne kan laves om til middelværdier over 60 sekunder når Lars opdaterer hele systemet. Lige nu er det medianværdi af de sidste 20 sekunder.

Start by tracing where weather measurements are collected, aggregated, and persisted, then compare the current calculation with Elsa's logbook note. Confirm whether the system currently uses the median of the last 20 seconds and document what would need to change for 60-second means; the issue does not name files or tests.

Written by the indexing model from the issue text.