feat: db pool + migration runner with idempotency

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

tests/helpers/db.js

@@ -0,0 +1,16 @@
+import { pool } from '../../lib/db/pool.js';
+
+export async function resetDb() {
+  await pool.query(`
+    DROP SCHEMA IF EXISTS public CASCADE;
+    CREATE SCHEMA public;
+    CREATE EXTENSION IF NOT EXISTS pgcrypto;
+    CREATE EXTENSION IF NOT EXISTS vector;
+  `);
+}
+
+export async function withClient(fn) {
+  const client = await pool.connect();
+  try { await fn(client); }
+  finally { client.release(); }
+}