Evaluate Supabase provisioning through Vercel

[zaridan]

Evaluate and document Supabase provisioning through Vercel.

Goal:

Reduce manual setup by using the Vercel/Supabase integration where it helps, without making Supabase or Vercel inseparable from Helpthread core.

Acceptance:

• Document what the Vercel Supabase integration can provision or sync automatically.
• Map Supabase-provided env vars to Helpthread's required env vars.
• Identify which values still need explicit operator input.
• Decide whether the alpha install path should require, recommend, or merely support the integration.
• Preserve the charter boundary: Vercel/Supabase are first-class deployment targets, not the definition of Helpthread.