chore(test): the DB lane runs on every core — each pytest-xdist worker gets its own database

backend/pytest.ini

@@ -7,4 +7,9 @@
 # not an unknown marker that quietly does nothing.
 testpaths = tests
 pythonpath = .
-addopts = -q -rs --strict-markers
+# `-p tests.db_per_worker`: under pytest-xdist (`-n`) every worker gets its own database — DATABASE_URL + `_gw<N>`,
+# created on first use — because the `db` fixture's TRUNCATE-per-test isolation assumes one process per database.
+# It is a plugin so it runs before conftest imports the app (whose settings read DATABASE_URL once). Serial: no-op.
+# The file is a verbatim copy of a shared tool (see its docstring); change the source, not the copy.
+addopts = -q -rs --strict-markers -p tests.db_per_worker
+db_per_worker_env = DATABASE_URL