Payload deploys can set host, urlPath, isolated and branchedDatabases #3043

Problem

A CI pipeline builds a component and uploads it, which makes it a payload deploy. The root-config fields that make an application host-routed, isolated or branched can only be set by a package deploy:

• host and urlPath fail validation without package (components/operationsValidation.js:609-610).
• isolated answers 400 (components/operations.js:535-541).
• branchedDatabases is accepted and then ignored (filed separately as a bug).

set_configuration can't write arbitrary application keys either; it allows only <name>_package and <name>_port. So there's no supported way to deploy a built bundle as an isolated, host-bound, branched application, which is what a PR preview is.

Proposal

Accept these fields on payload deploys, and publish them in the component's root-config entry as an effect of the activation transaction (#2315 step 3), the same way package deploys do. A payload redeploy that omits a field keeps the existing value, as isolated already does.

Done when

• harper deploy project=shop-pr-12 host=fix-cart.shop.example.com isolated=true branchedDatabases=true from a directory produces an isolated, host-bound, branched application.
• A rejected or rolled-back build leaves these fields unchanged, with the same guarantees step 3 gives package.

Part of #3042.

Update, 2026-10-07

• Document isolated applications, branched databases, and per-PR CI previews documentation#715 documents the workaround this issue removes. An app that needs a build publishes each PR commit to a registry and deploys it as a package, which can set isolated and host; a payload upload can't.
• A package deploy's branchedDatabases is ignored at load, so the application runs on its base databases #3071: a package entry's branchedDatabases is also ignored, at load. So branches through any deploy path need both fixes.