feat[worker]: add headless/API-only deployment mode

[ignaciojimenez]

Adds a HEADLESS_MODE environment variable for API-only deployments.

When enabled, the index route serves a minimal landing page with curl
usage instructions instead of the React frontend. URL redirect (/u/)
and article rendering (/a/) return 403, since they depend on the UI.
All read/write/delete API routes are unaffected.

Includes tests and documentation updates.

[SharzyL]

I wonder what is headless mode used for. If it is used for CLI-only cases, should it return plain text instead of an HTML page?

[ignaciojimenez]

Good question. My main use case is using this as a CRUD interface to Cloudflare KV/R2 storage — a lightweight paste service without the React frontend, CLI-focused or embedded in infrastructure where the SPA isn't needed. I think others might find it useful for the same reason: a simple, deployable API layer on top of Cloudflare storage.

Headless mode disables the upload UI, URL redirects (/u/), and article rendering (/a/), while keeping paste CRUD and the display page (/d/) intact.

For the landing page: you're right that plain text would be more consistent with the CLI-only philosophy. I can switch it to return text/plain — something like:

Pastebin (API-only mode)

Upload:  curl -Fc=@file.txt https://example.com
Docs:    https://example.com/api

Would that work, or do you have a different preference?

(Also pushed a fix for the failing CI — was a missing .js extension on a test import.)