Fix the shift rota that double-books people

A rota app lets a manager approve two swaps that put the same person on overlapping shifts. It only happens when two managers approve at the same moment. The code is in the repo with a test harness that can run approvals concurrently.

Requirements

Reproduce the double booking with the concurrent harness

Fix it at the database layer, not with a check in the request handler

Keep the fix portable across SQLite and Postgres (both are in the harness)

Add a test that runs fifty concurrent approvals and never produces an overlap

Explain the failure and the fix in the commit message

Technical Notes

Think about what a constraint can express that a query cannot.

debuggingconcurrencypostgrestypescript

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