test(db/postgres): serialise the integration files under vitest 4

The lane's files share one database and schemaMigrations drops its
tables mid-suite, so they must run one at a time. That was serialised
with poolOptions.forks.singleFork, which vitest 4 removed; the option
now silently no-ops and the files ran concurrently, so the drop raced
the other suites (relation "repos" does not exist). fileParallelism
false is the vitest 4 equivalent.

Author: dcoric

vitest.config.integration.postgres.ts

@@ -24,11 +24,11 @@ export default defineConfig({
     hookTimeout: 10000,
     setupFiles: ['test/setup-integration-postgres.ts'],
     pool: 'forks',
-    poolOptions: {
-      forks: {
-        singleFork: true,
-      },
-    },
+    // The files share one database and some of them drop and recreate its
+    // tables, so they must not run concurrently. vitest 4 removed the old
+    // poolOptions.forks.singleFork switch (it silently no-ops), so file
+    // parallelism is disabled explicitly.
+    fileParallelism: false,
     env: {
       NODE_ENV: 'test',
       RUN_POSTGRES_TESTS: 'true',