fix: give each test worker its own SQLite database

Parallel Vitest workers were opening data/pichost.db together, so host-isolation failed with database is locked.

server/test/setup.ts

@@ -0,0 +1,22 @@
+import { mkdtempSync, rmSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { afterAll, beforeAll } from 'vitest'
+import { closeDb } from '../utils/db'
+
+const workerId = process.env.VITEST_POOL_ID ?? String(process.pid)
+const dataDir = mkdtempSync(join(tmpdir(), `pichost-test-${workerId}-`))
+process.env.DATA_DIR = dataDir
+
+beforeAll(() => {
+  closeDb()
+})
+
+afterAll(() => {
+  closeDb()
+  try {
+    rmSync(dataDir, { recursive: true, force: true })
+  } catch {
+    // Windows may still hold the sqlite file for a moment after close
+  }
+})