publish endpoint

Closes #10 (closed). The connection string split on the scheme of the endpoint URL; the remote is now declared through rclone's environment variables. Proven on this branch (pipeline 2969482812, 13 files uploaded), then locked back to main.

Before merging: the branch was protected so the protected variables reached its pipeline. Unprotect it afterwards:

glab api -X DELETE "projects/corvusobserver%2Fdocs/protected_branches/publish_endpoint"

Found while testing: the bucket serves /index.html but not /, because an R2 custom domain does not resolve directory paths to index.html. That is a platform change (a Cloudflare URL rewrite rule on the hostname), tracked separately.