feat: add versioned migrations and kysely query layer

backend/libs/database/test/migration-runner.integration-spec.ts

@@ -0,0 +1,31 @@
+import { Pool } from 'pg';
+import { runMigrations } from '../../../apps/api/src/migration';
+
+describe('runMigrations (integration)', () => {
+  it('applies all pending migrations idempotently against a real database', async () => {
+    await runMigrations();
+    await runMigrations(); // must be safe to run twice
+
+    const pool = new Pool({ connectionString: process.env.DATABASE_URL });
+    try {
+      const tables = await pool.query<{ table_name: string }>(
+        `select table_name from information_schema.tables where table_schema = 'public'`,
+      );
+      const names = tables.rows.map((r) => r.table_name);
+      for (const expected of [
+        'users',
+        'user_preferences',
+        'trips',
+        'trip_settings',
+        'trip_members',
+        'trip_invitations',
+        'travelers',
+        'trip_preference_overrides',
+      ]) {
+        expect(names).toContain(expected);
+      }
+    } finally {
+      await pool.end();
+    }
+  });
+});