Concurrency & async
How JavaScript's event loop actually works, what async/await desugars to, the race conditions you didn't think you had, and the idempotency patterns that let "exactly once" stop being a lie.
Distributed systems
The conceptual primer — why distributed systems are different from a single machine, the CAP theorem in working-engineer terms, consistency models, leader election, distributed transactions, and the saga pattern that replaces them.