Add a generic job scheduler

Also rewrite deletion of old sends using the job scheduler.

.env.template

@@ -56,6 +56,19 @@
 # WEBSOCKET_ADDRESS=0.0.0.0
 # WEBSOCKET_PORT=3012
 
+## Job scheduler settings
+##
+## Job schedules use a cron-like syntax (as parsed by https://crates.io/crates/cron),
+## and are always in terms of UTC time (regardless of your local time zone settings).
+##
+## How often (in ms) the job scheduler thread checks for jobs that need running.
+## Set to 0 to globally disable scheduled jobs.
+# JOB_POLL_INTERVAL_MS=30000
+##
+## Cron schedule of the job that checks for Sends past their deletion date.
+## Defaults to hourly. Set blank to disable this job.
+# SEND_PURGE_SCHEDULE="0 0 * * * *"
+
 ## Enable extended logging, which shows timestamps and targets in the logs
 # EXTENDED_LOGGING=true