ci: run Postgres integration tests in CI

[0dillon]

Summary

The internal/store Postgres integration tests are currently hidden behind the postgres_integration build tag and only run locally. We should spin up a Postgres service container in GitHub Actions and run these tests automatically.

Acceptance Criteria

• Update .github/workflows/ci.yml to include a Postgres service.
• Add a step to run go test -tags postgres_integration ./internal/store/.

Tech Stack

GitHub Actions, Docker, Go