feat(slice-1): backend skeleton, deps, and import-linter layer contract

Four-layer package (main > domains > platform) per SD-0001 §6.2; GraphQL api layer
deferred. main.py is a stub so the top layer exists from the start (fleshed out in
the /healthz task). lint-imports green on the skeleton.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/pytest.ini

@@ -0,0 +1,7 @@
+[pytest]
+testpaths = tests
+python_files = test_*.py
+addopts = -q
+# Put backend/ (this rootdir) on sys.path so tests import the `app` package under
+# pytest's default prepend import mode (tests/ has no __init__.py).
+pythonpath = .