test(orders): stop integration files racing on the shared test database

The integration suite shares one personal_test database and helpers.resetDB()
TRUNCATEs it with CASCADE, which reaches expense_metadata via the transactions
FK. With file parallelism on, queries.test.ts and participants.test.ts were
truncating rows out from under order-ingestion.test.ts mid-test, so a different
set of assertions failed on every run — including I6 (credits), I7
(idempotency) and I11 ([Family]), the three invariants the suite exists to
prove. Serialise the files.

Suite was reported 31/31 green; observed 29/31 then 28/31 on consecutive runs.
Now 31/31 on three consecutive runs.

vitest.integration.config.ts

@@ -20,5 +20,10 @@ export default defineConfig({
     environment: "node",
     include: ["src/__tests__/integration/**/*.test.ts"],
     pool: "forks",
+    // Every integration file shares the one `personal_test` database, and
+    // helpers.resetDB() TRUNCATEs it (CASCADE reaches expense_metadata).
+    // Run files one at a time so a sibling's reset cannot delete rows another
+    // file just inserted — that raced non-deterministically across I6/I7/I11.
+    fileParallelism: false,
   },
 });