Rename expectedKey to key and make it optional

[Copilot]

Renamed the expectedKey parameter to key and made authentication optional. When no key is provided, bearer auth middleware is not applied.

API Changes

• Parameter rename: expectedKey → key in createApp()
• Type change: key is now optional (key?: string)
• Behavior: Bearer auth middleware only applied when key is provided

Usage

// With authentication
const app = createApp({
  key: "my-secret",
  proxyUri: "http://proxy.example.com:8080"
});

// Without authentication
const app = createApp({
  proxyUri: "http://proxy.example.com:8080"
});

Environment Variables

• PROXY_FETCH_KEY remains the environment variable name (backward compatible)
• Now optional; server starts without authentication when omitted
• PROXY_URI is the only required variable

Modified Files

• src/app.js: Parameter rename, conditional middleware application
• src/server.js: Removed key validation requirement
• tests/server.test.js: Added tests for unauthenticated mode
• README.md: Updated to reflect optional authentication

Original prompt

Rename expectedKey to key, and make it optional.

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