feat(cli): add Autumn payment provider

[kylegrahammatzen]

Add Autumn (docs | github | sdk) as a payment provider option alongside Polar.

Autumn supports Convex backend and has a CLI setup workflow similar to database providers like Neon/Turso.

What I'm planning to add

• Add Autumn to the payment selection prompt
• CLI integration with automatic/manual setup modes (similar to database provider setup)
  • Automatic: runs npx atmn init during scaffolding, opens browser for auth
  • Manual: user runs npx atmn init later
• Templates for Convex backend integration (autumn.ts, AutumnWrapper.tsx)
• Basic client setup for other backends (Hono, Express, Elysia, Fastify, Next.js)
• Better Auth/Clerk integration for customer identification
• Web frontend required (React, Vue, Svelte, Solid)
• Installs autumn-js for all backends, @useautumn/convex for Convex

I can work on the PR, just looking to get approval if this aligns with the CLI's goals.