Side note -- I know a lot of early YC startups like to play things fast and loose, but you really should compensate your engineers for after-hours emergencies if they are already working 40 hour weeks. Morally and employee-retention-wise it's the obvious thing to do, but beyond that in certain states and jurisdictions you can easily run afoul of local labor laws if you try to require employees to do things outside of their regular hours without compensation. Another very important angle to consider is you want to incentivize your employees to answer the call to deal with these issues. If there is no incentive structure in place, they might not take it seriously, which in itself can be a security and stability risk that could threaten things like your SOC-2 compliance, if you have it.
When I worked as a full time DoD scientist, there was a very set-in-place system for dealing with these situations, and it was normal to pay double overtime for any hours spent by an employee on after hours emergencies. This is the right way to do it. Pre-series B startups largely don't do this, but once you get to series B and C suddenly it becomes a thing because companies realize they either have to legally, or have to to prevent their employees from churning and to protect themselves from people not showing up to put out the fire.
Just do it, and do it early. Do it before Series A. That's the advice I give my consulting clients, and the approach I take with my own companies. By compensating your employees for this time you also take what would be a red flag for many would-be employees and turn it into an exciting perk.