fix: serialize test file execution to avoid cross-file DB races

Each test file gets its own throwaway Postgres database by mutating
process.env.DATABASE_URL in tests/setup.ts's beforeAll. That per-file
DB switch is not safe under Vitest's default parallel file execution:
timing between two files' beforeAll/query calls can interleave enough
that both end up issuing real queries against the same throwaway DB.

This was invisible before because no two test files shared tables
with a real FK between them. It became visible once Request gained a
real FK to Employee: tests/employees.test.ts and tests/requests.test.ts
started intermittently deleting/reading each other's rows (FK
violations on deleteMany, wrong row counts), reproducible on every
run. Setting fileParallelism: false makes each file run to completion
before the next starts, so the per-file throwaway DB is genuinely
private. Confirmed: 31/31 tests pass reliably under plain npm test
(previously reproducibly 4 failing).

vitest.config.ts

@@ -10,5 +10,18 @@ export default defineConfig({
   test: {
     environment: "node",
     setupFiles: ["./tests/setup.ts"],
+    // Each test file spins up its own throwaway Postgres database by
+    // mutating process.env.DATABASE_URL in tests/setup.ts's beforeAll (see
+    // that file). That per-file DB switch is not safe under Vitest's default
+    // parallel file execution: two files' beforeAll/query timing can
+    // interleave such that both end up issuing real queries against the
+    // same throwaway DB (confirmed empirically once Request gained a real
+    // FK to Employee — tests/employees.test.ts and tests/requests.test.ts
+    // started intermittently deleting/reading each other's rows, causing
+    // spurious FK-violation and row-count failures that disappear when run
+    // sequentially). Disabling file parallelism trades a bit of wall-clock
+    // test time for correctness — each file's throwaway DB is now genuinely
+    // private for the duration of that file's run.
+    fileParallelism: false,
   },
 });