Add a weekly dump of the Weather database

weather-dump.timer runs weather-dump weekly, keeping the two newest ISO weeks in
~/.local/share/ens-forecast/dumps, which simple_backup then copies; the login comes
from ~/.config/ens-forecast/dump.cnf (not tracked).

.config/systemd/user/weather-dump.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Dump the Weather database to ~/.local/share/ens-forecast/dumps
+StartLimitIntervalSec=2h
+StartLimitBurst=4
+
+[Service]
+Type=oneshot
+ExecStart=%h/.local/bin/weather-dump
+# MariaDB not up yet (e.g. caught up right after boot)? retry a few times
+Restart=on-failure
+RestartSec=5min