Refactor worker into spec-driven server architecture

[arielpollack]

Summary

This PR refactors md.page from a single-file Worker into a spec-driven server architecture while preserving the existing public behavior and Cloudflare Worker runtime.

The main goals were:

• split request handling into clear route modules
• separate business logic into services
• move pure helpers into domain modules
• isolate HTML/static rendering concerns
• document product, API, architecture, and engineering decisions
• add an AGENTS.md guide so future contributors and agents can navigate the codebase quickly

What Changed

Architecture

• Replaced the large monolithic src/index.ts implementation with a thin Worker entrypoint
• Added src/app.ts to assemble the application
• Added routing primitives under src/server/
• Split handlers into:
  • src/routes/api-routes.ts
  • src/routes/page-routes.ts
  • src/routes/static-routes.ts
• Split workflows into services under src/services/
• Moved pure logic into src/domain/
• Moved templates and static assets into src/presentation/

Spec-driven docs

Added a small spec set under docs/spec/ covering:

• product constraints
• API contract
• architecture boundaries
• engineering decisions

Also added AGENTS.md with a code map and instructions for future contributors/agents.

Dev workflow

• Added wrangler.toml so bun dev and wrangler dev work with the refactored entrypoint
• Updated README.md and CONTRIBUTING.md to reference the new architecture and docs

Behavior / Compatibility

This refactor intentionally preserves the existing public contract, including:

• POST /api/publish
• POST /api/event
• GET /:id
• GET /og/:id.png
• landing page, privacy page, favicon, logo, and fallback OG image
• existing validation and rate limiting behavior
• existing response formats and key headers

Verification

• npm run typecheck
• npm test

All tests passed after the refactor.

Notes

This PR is mostly architectural and documentation-focused. It should make future feature work much easier without changing the product surface area.