On low balance, top-up or notify
On start up, check for users with low balance and NOTIFY about them. LISTEN for
such notifications and process by either sending a low-balance warning message
or else attempting an auto-top-up as configured.
Using NOTIFY/LISTEN because then we can NOTIFY after any INSERT that leaves the
balance too low (using a trigger). Doing the sync on start-up in case we missed
a NOTIFY during any downtime. Using the Redis lock to prevent spamming a
low-balance user in case of many restarts or if they have many small
transactions happen in one day.