For whoever picks this up (probably me in a month):
- export.py reads readings.db and writes readings.csv, runs from systemd nightly
- garage sensor reports every 15 min, porch every 5, the CSV has both
- the --since flag works but isn't used by the service yet
- TODO: rotate old CSVs, currently they pile up in the same directory
Config is in exporter.ini next to the script.