feat(proxy): add environment config and request handling

- Introduce environment-based configuration for proxy settings
- Add middleware for API key authentication
- Implement request forwarding to LanguageTool backend
- Set up server startup and routing logic

src/ltProxyAuth.ts

@@ -0,0 +1,21 @@
+import { Middleware } from 'http-kernel/Types/mod.ts';
+import { Env } from './env.ts';
+
+/**
+ * Middleware that checks for a valid API key via ?apiKey=... query/form param.
+ * Rejects request with 403 if the key is missing or invalid.
+ */
+export const authMiddleware: Middleware = async (ctx, next) => {
+    const key = ctx.query.apiKey;
+
+    // Support both ?apiKey=... and form body with apiKey=...
+    const extractedKey = Array.isArray(key) ? key[0] : key;
+
+    if (!extractedKey || !Env.apiKeys.includes(extractedKey)) {
+        return new Response('Forbidden – Invalid API key', { status: 403 });
+    }
+
+    return await next();
+};
+
+export { authMiddleware as ltProxyAuth };