feat: self-hosted API docs page at / and /docs

[ronaldtse]

Summary

• the API's webpage is deployed content of the worker itself: GET /docs (always HTML) and GET / with content negotiation — browsers get the docs page, API clients keep the existing JSON directory unchanged
• self-contained HTML (inline CSS, system fonts, no external assets): endpoint table, quick-start curl, OpenAPI/model/project links
• api.interscript.org becomes self-describing; deployed docs live in this repo per the deployment-content ownership rule

Test plan

• content negotiation + /docs tests (TDD: red -> green)
• 25/25 suite, typecheck, lint, format, build
• e2e re-run against staging after deploy