feat: db pool + migration runner with idempotency

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

tests/db/migrate.test.js

@@ -0,0 +1,28 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+import { resetDb, withClient } from '../helpers/db.js';
+import { migrateUp } from '../../lib/db/migrate.js';
+
+describe('migrate', () => {
+  beforeAll(async () => { await resetDb(); });
+
+  it('creates schema_migrations table on first run', async () => {
+    await migrateUp();
+    await withClient(async (c) => {
+      const { rows } = await c.query(
+        `SELECT to_regclass('public.schema_migrations') AS t;`
+      );
+      expect(rows[0].t).toBe('schema_migrations');
+    });
+  });
+
+  it('is idempotent — second run is a no-op', async () => {
+    await migrateUp();
+    await migrateUp();
+    await withClient(async (c) => {
+      const { rows } = await c.query(
+        `SELECT count(*)::int AS n FROM schema_migrations;`
+      );
+      expect(rows[0].n).toBeGreaterThanOrEqual(0);
+    });
+  });
+});