Break: concurrent writes corrupt a balance; pg_dump backup and restore

This lesson is still being written.

The plan for today is above. Every lesson in the course follows the same contract:

1. What problem does it solve, and what did people do before it existed?
2. How does it work underneath, and what does it hide from the developer?
3. When should it be used, when should it not, and what does it cost?
4. Hands-on: build the smallest working thing, then break it on purpose and debug it.
5. Security: what can go wrong, who could attack it, what could they reach, how would we detect, prevent and recover?
6. AI check (from Phase 1): ask an assistant to explain or produce the same thing, then find what it got wrong.

This week's deliverable: Project 3