feat: add Vercel serverless functions for image and news search

[bytebinders]

What changed

• Added Vercel serverless endpoints api/images.ts and api/news.ts with parity to Express /images and /news routes.
• Implemented full x402 payment requirements (0.001 USDC / 10000 stroops), CORS preflight headers, and replay protection.
• Added comprehensive unit and integration tests in api/images.test.ts and api/news.test.ts.
• Configured coverage thresholds for both new modules in vite.config.ts.
• Updated documentation and project structure in README.md, CONTRIBUTING.md, and api/index.ts.

Why it changed

• Resolves endpoint disparity where serverless deployments previously only exposed /api/search while Express supported paid image and news searches.

How it was verified

• Verified parameter sanitization, error responses (400, 402, 405, 502), and response data structures across both endpoints.
• Validated tests and cross-runtime constants alignment (STELLAR_NETWORK, USDC_CONTRACT, AMOUNT_STROOPS, AMOUNT_USDC).

Closes #217

[vercel]

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[AbuJulaybeeb]

fix failing ci