feat(preview): use build server for preview deployments

[tanmay-pathak]

What is this PR about?

This PR routes build steps to the effective build server (buildServerId if a remote build server is set, falling back to serverId) across regular deploys, rebuilds and preview deploy flows. We create a build-scoped application object with serverId set to the build server so build helpers generate correct paths/remote context, while deployment execution stays on the app server.

Checklist

Before submitting this PR, please make sure that:

• You created a dedicated branch based on the canary branch.
• You have read the suggestions in the CONTRIBUTING.md file
• You have tested this PR in your local instance.
  • tested deploy of main branch using Dockerfile to build on the same server
  • tested deploy of main branch using Dockerfile to build using a remote build server
  • tested deploy of a preview branch using Dockerfile to build on the same server
  • tested deploy of a preview branch using Dockerfile to build using a remote build server
  • verified build actually happens on the respective servers by looking at the CPU utilization on host/remote servers

Issues related

Closes #3450

Screenshots

Screen.Recording.2026-01-14.at.7.30.21.PM.mov