fix: close PGlite handle after tests to resolve exit code 100

PGlite's WASM worker kept an open async handle, causing Bun to detect
a resource leak and exit with code 100 despite all tests passing.
Adds a preload script that closes the cached client via afterAll.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/helpers/db.ts

@@ -7,7 +7,7 @@ import * as schema from "../../src/db/schema.ts";
 type Db = ReturnType<typeof drizzle<typeof schema>>;
 
 // Cache: one PGlite instance per test file (per worker)
-let cachedClient: PGlite | null = null;
+export let cachedClient: PGlite | null = null;
 let cachedDb: Db | null = null;
 
 async function getOrCreateDb(): Promise<Db> {