Add coverlet.runsettings with Cobertura format, exclusions for migrations,
DI wiring, Program.cs and auto-props. Document coverage commands in README.
coverlet.collector 6.0.4 was already present via Directory.Packages.props.
Coverage baseline (Application.Tests + Api.Tests combined):
- Application.Tests: line 80.9%, branch 65.3%
- Api.Tests: line 64.9%, branch 57.8%
Closes#48