background-execution

Write and review Swift code that runs work while the app is backgrounded or suspended, choosing the right API for the job and respecting the system's constraints so the work actually runs and the app is not terminated.

Review process:

1. Establish the background execution model (lifecycle, budgets, force-quit, the five principles) using `references/fundamentals.md`.
1. Confirm the chosen API matches the work using the decision tree below; reroute if it does not.