chore: add test infrastructure with conftest for import paths

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pytest.ini

@@ -0,0 +1,2 @@
+[pytest]
+testpaths = tests

tests/conftest.py

@@ -0,0 +1,9 @@
+"""Test configuration — adds package root to sys.path so bare imports work."""
+import sys
+from pathlib import Path
+
+# Add the package root so `from stats_db import StatsDB` works in tests
+# without requiring the full package to be installed.
+_PACKAGE_ROOT = Path(__file__).resolve().parent.parent
+if str(_PACKAGE_ROOT) not in sys.path:
+    sys.path.insert(0, str(_PACKAGE_ROOT))