fix: isolate Prisma Client per test file, re-enable fileParallelism

Root-cause fix for cross-file DB pollution in the vitest suite, which
surfaced as intermittent FK-violation failures once the suite hit 12
test files. tests/setup.ts previously mutated the global
process.env.DATABASE_URL and relied on src/lib/prisma.ts's singleton
picking it up, which is racy across files sharing a worker process
(process.env isn't reset between files even though Vitest resets each
file's module registry). fileParallelism: false only masked this.

Now each test file's own tests/setup.ts creates its own throwaway
Postgres database and vi.mock()s "@/lib/prisma" with a dedicated
PrismaClient constructed via the explicit datasourceUrl option, so no
mutable state is shared between files and fileParallelism can go back
to its default (true).

Verified with 4 consecutive full `npm test` runs (327/327 passing each
time) and no leftover throwaway databases after each run.

vitest.config.ts

@@ -20,18 +20,12 @@ export default defineConfig({
     // second worktree existed.
     include: ["tests/**/*.test.ts"],
     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,
+    // Each test file gets its own throwaway Postgres database and its own
+    // dedicated PrismaClient, wired up via a `vi.mock("@/lib/prisma", ...)`
+    // factory in tests/setup.ts (not by mutating the ambient
+    // process.env.DATABASE_URL, which used to be shared/racy across files —
+    // see the long comment at the top of tests/setup.ts for the history).
+    // With no mutable state shared between files, parallel file execution
+    // (Vitest's default) is safe again.
   },
 });