Define one-command Vercel and Supabase setup path

[zaridan]

Define the simplest credible path for a new operator to spin up Helpthread on the first-class serverless stack. Acceptance: document required services, environment variables, OAuth/IMAP/SMTP choices, setup order, and what can be automated later.

[zaridan]

Added focused Vercel installability issues under this umbrella:

• Single Vercel project deployment target
• Deploy with Vercel alpha path
• Migration automation/readiness
• Supabase provisioning/integration mapping
• Future Vercel Marketplace guided install

These break "one-click install" into the practical sequence:

1. make the deploy target simple;
2. add a Deploy Button;
3. make setup/migrations safe;
4. reduce Supabase/env friction;
5. later build a true Vercel Marketplace integration.