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/server.ts

@@ -8,6 +8,7 @@ import { listModels, sendChat, sendChatStream } from './chatwrapper';
 import { mapRequest, mapResponse, mapStreamChunk } from './mapper.js';
 import { RequestBody, GeminiResponse, GeminiStreamChunk, Part } from './types';
 import { config } from './config';
+import { isAuthorized } from './auth';
 
 // ==================================================================
 // Server Configuration
@@ -97,6 +98,10 @@ http
       return;
     }
 
+    if (!isAuthorized(req, res)) {
+      return;
+    }
+
     // Route for listing available models.
     if (pathname === '/v1/models' || pathname === '/models') {
       res.writeHead(200, { 'Content-Type': 'application/json' });