SaaS Waitlist Backend

Picks one of four adapters and wires it into the generated landing page.

Adapter selection logic

In priority order:

1. User explicitly chose one in chat → honor it.
2. SUPABASE_URL + SUPABASE_ANON_KEY in env → use Supabase (table waitlist_signups).
3. RESEND_API_KEY in env → use Resend Audiences.
4. LOOPS_API_KEY in env → use Loops.
5. No env hints → default to local JSON (sites/<slug>/.data/waitlist.json) — zero external dependencies, perfect for early validation.

After picking, ask the user once for confirmation in chat, then proceed. Don't pause again.

What you write per adapter