feat(auth): add api key authentication

Implement API key authentication by introducing a new auth module.
Update configuration and .env.example to support API key setup, and add
authorization checks in the server endpoints.

src/config.ts

@@ -23,4 +23,10 @@ export const config = {
    * @type {boolean}
    */
   VERBOSE: Boolean(process.env.VERBOSE ?? true),
+  /**
+   * The API key for securing the server.
+   * If not set, the server will be public.
+   * @type {string | undefined}
+   */
+  API_KEY: process.env.API_KEY,
 };