Database size reduction and automatic backup

Added daily background job deleting speed_graph_data older than 1 year. Also added backup timer to systemd, which will create weekly dumps in the /backup directory.

The gzip'd size of a single speed_graph_data item in the test table is roughly 6250B. Keeping only relatively recent data does not break anything in particular and we save a lot of disk space. It might be even better to lower the retention rate to 6 months as the data is not really meaningful that far into the history.