feat(examples): add basic Express app with x402charity middleware

[bimakw]

What

Add a minimal Express example app under examples/express-basic/ that demonstrates the x402charity middleware in action, resolving #2.

Changes

examples/express-basic/
├── package.json    # Dependencies: express + x402charity
├── README.md       # Setup instructions and configuration docs
└── index.js        # Simple Express app with x402charity middleware

How it works

1. Install dependencies with npm install
2. Set PRIVATE_KEY env var (Base Sepolia testnet wallet with USDC)
3. Run npm start
4. Every HTTP request triggers a $0.001 USDC micro-donation via the x402 protocol

Features

• 3 demo routes: / (info), /hello (greeting), /health (uptime)
• Non-blocking: Donations happen asynchronously, requests are never delayed
• Testnet safe: Configured for Base Sepolia by default
• Production ready: README includes instructions for switching to mainnet

Testing

cd examples/express-basic
npm install
export PRIVATE_KEY=0xYOUR_TEST_KEY
npm start

# In another terminal:
curl http://localhost:3000/hello

Closes #2

[vercel]

@bimakw is attempting to deploy a commit to the shawnpang's projects Team on Vercel.

A member of the Team first needs to authorize it.