Add CI job running the tests against a real PostgreSQL service

[Julien Dubois (jdubois)]

The existing build job runs the suite against in-memory H2, so the org.postgresql:r2dbc-postgresql driver the sample actually ships with is never exercised in CI. A driver-level regression would go unnoticed.

Adds a build-postgres job that reruns the same tests against a postgres:16 service container, overriding only the connection settings via environment variables (OS env outranks application.properties in Spring Boot's property order, so no code or test changes are needed).

The build job is left untouched so its check name is unchanged and any branch protection rules keep working.

Verified locally against a postgres:16 container matching the service block: 5/5 tests pass and schema.sql creates the todo table on PostgreSQL. Confirmed the override actually takes effect by pointing the URL at a dead port, which fails all 5 tests rather than silently falling back to H2.

Purpose

• ...

Does this introduce a breaking change?

[ ] Yes
[ ] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Get the code

git clone [repo-address]
cd [repo-name]
git checkout [branch-name]
npm install

• Test the code

What to Check

Verify that the following are valid

• ...

Other Information