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/env.ts

@@ -0,0 +1,63 @@
+/**
+ * Environment configuration for lt-auth-proxy.
+ * All properties are lazily evaluated and cached on first access.
+ */
+
+export class Env {
+    private static _proxyHost?: string;
+    private static _proxyPort?: number;
+    private static _apiKeys?: string[];
+    private static _ltServerHost?: string;
+    private static _ltServerPort?: number;
+
+    private static getEnv(key: string, required = false): string | undefined {
+        const value = Deno.env.get(key);
+        if (required && (!value || value.trim() === '')) {
+            throw new Error(`Missing required environment variable: ${key}`);
+        }
+        return value;
+    }
+
+    /** Hostname for the proxy (default: 0.0.0.0) */
+    static get proxyHost(): string {
+        if (this._proxyHost === undefined) {
+            this._proxyHost = this.getEnv('PROXY_HOST') || '0.0.0.0';
+        }
+        return this._proxyHost;
+    }
+
+    /** Port for the proxy (default: 8011) */
+    static get proxyPort(): number {
+        if (this._proxyPort === undefined) {
+            this._proxyPort = Number(this.getEnv('PROXY_PORT') || 8011);
+        }
+        return this._proxyPort;
+    }
+
+    /** List of allowed API keys (required) */
+    static get apiKeys(): string[] {
+        if (this._apiKeys === undefined) {
+            const raw = this.getEnv('API_KEYS', true)!;
+            this._apiKeys = raw.split(',').map((k) => k.trim()).filter((k) =>
+                k.length > 0
+            );
+        }
+        return this._apiKeys;
+    }
+
+    /** Hostname of the LanguageTool backend (default: localhost) */
+    static get ltServerHost(): string {
+        if (this._ltServerHost === undefined) {
+            this._ltServerHost = this.getEnv('LT_SERVER_HOST') || 'localhost';
+        }
+        return this._ltServerHost;
+    }
+
+    /** Port of the LanguageTool backend (default: 8010) */
+    static get ltServerPort(): number {
+        if (this._ltServerPort === undefined) {
+            this._ltServerPort = Number(this.getEnv('LT_SERVER_PORT') || 8010);
+        }
+        return this._ltServerPort;
+    }
+}