Great stack! I'm doing a similar approach for my latest project (kavla.dev) but using fly.io and their suspend feature.

Scaling to zero with database persistence using litestream has cut my bill down to $0.1 per month for my backend+database.

Granted I still don't have that many users, and they get 200ms of extra latency if the backend needs to wake up. But it's nice to never have to worry about accidental costs!