Phase 1 of AUDIT-2026-Q3.md item P1.2.
tests/ was not a Python package, so downstream imports
(`from tests.unit.x import y`, testcontainers config loading,
pytest --import-mode=importlib) failed without PYTHONPATH=.
Creating __init__.py in tests/, tests/unit/, tests/integration/
makes the suite a proper package. pytest collection already worked
via rootdir discovery (886 tests collected either way), but
*importing* test modules will now work.
Also seeds tests/integration/conftest.py with a Phase-1 stub
fixture. Phase 3 (testcontainers) replaces it.
Full fix lands in P1.3 (setuptools.packages = ["app"] +
tools.setuptools.package-data in pyproject.toml) — without that,
`pip install -e .` does not expose the app package.
