feat: add dynamic OpenAPI/Swagger documentation

[monil80]

Summary

• GET /docs — Swagger UI (CDN, no new dependencies)
• GET /openapi.json — OpenAPI 3.0 generated from db.json at runtime
• --no-swagger and createApp({ swagger: false }) to disable

Test plan

• npm test
• npm run lint
• npx json-server fixtures/db.json → http://localhost:3000/docs
• npx json-server --no-swagger fixtures/db.json → /docs returns 404

[tamjid14]

This looks like a useful change. One thing I would suggest is adding regression tests for both the new behavior and the existing behavior, so the change does not accidentally break current filtering syntax.

For example, if the PR changes filtering behavior, it would be useful to test:

GET /posts?title=hello

and also any equivalent _where or operator-based syntax that should still work.

That would make the change safer and easier to review because maintainers can see exactly which behavior is intended to change and which behavior should stay compatible.