test: add jest/supertest infrastructure and make server.js testable

Made-with: Cursor

tests/jest.setup.js

@@ -0,0 +1,4 @@
+process.env.DB_PATH = ':memory:';
+process.env.JWT_SECRET = 'test-jwt-secret-do-not-use-in-prod';
+process.env.ADMIN_KEY = 'test-admin-key';
+process.env.PORT = '0';