feat: reusable workflow for pytest (unit and integration)

[MarkCesium]

Summary

Existing python-lint.yml covers ruff + mypy. Test issues (a6, p6, d9, co8, g5) need a matching reusable workflow to run pytest with testcontainers support in CI.

Changes

• .github/workflows/python-test.yml alongside python-lint.yml.
• Sets up Docker (testcontainers needs a daemon), runs uv sync --group dev, runs uv run pytest --cov --cov-report=xml.
• Uploads coverage XML as artefact.
• Inputs: python-version (default 3.14), test-path (default tests/), coverage-min (default 70).

Verification

• Backend service CI (e.g. places) calls PlaceBrain/workflows/.github/workflows/python-test.yml@main and passes on the existing tests.