docs(tests): add README documenting CENTRAL_TEST_DB_DSN env var

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/README.md

@@ -0,0 +1,18 @@
+# Central Tests
+
+## Test Database
+
+Some tests (notably `test_config_store.py`) require a real PostgreSQL database.
+By default, tests connect to:
+
+```
+postgresql://central_test:testpass@localhost/central_test
+```
+
+If your test database uses different credentials, set the `CENTRAL_TEST_DB_DSN`
+environment variable:
+
+```bash
+export CENTRAL_TEST_DB_DSN="postgresql://myuser:mypass@localhost/mydb"
+uv run pytest tests/test_config_store.py
+```