Add JSON API for x402 listings

[LubuSeb]

Shipyard bounty: bnt_Xhm2SMIm

Summary

• Add a Node/TypeScript JSON API for the x402 directory.
• Implement GET /api/listings with q, category, limit, and offset support.
• Implement GET /api/listings/:id and GET /api/categories.
• Seed the directory with static records from public x402.direct service metadata, marked as indexed records.
• Add robust JSON errors for missing listings, malformed encoded IDs, unsupported methods, and unknown routes.
• Add tests covering categories, search, filtering, pagination, detail lookup, 400/404/405 behavior.

Scope notes

• The seed data is static and in-memory.
• This PR does not add database persistence, live endpoint health checks, or payment execution.
• Requires Node.js 20 or newer.

Verification

• npm test
• npm run build
• npm audit --json reports 0 vulnerabilities
• Live local API checks passed for /api/categories, /api/listings, filtered listings, search, detail lookup, malformed ID 400, and POST 405 with Allow: GET.