Make Helpthread deploy as one Vercel project

[zaridan]

Make Helpthread deploy as one Vercel project for external operators.

Current state:

• The engine/API and web app are separate deployable surfaces.
• That creates duplicated env setup and makes a clean Deploy Button path harder.

Goal:

One Vercel deployment should serve:

• the web UI
• /api/v1/**
• internal cron routes
• Gmail callback/webhook routes

Acceptance:

• A fresh operator can deploy one Vercel project instead of separate API and web projects.
• API and UI share one canonical public origin unless explicitly configured otherwise.
• Cron routes continue to work under the same project.
• The local dev workflow remains usable.
• Docs explain the migration from the current split deployment model.
• The change does not introduce any hosted Helpthread or Resonant IQ data path.