Build B2B SaaS foundation

[im-anishraj]

Summary

• Builds the B2B SaaS foundation for InstaReply: email magic-link auth, workspace tenancy, Instagram integration, billing, webhook processing, queue worker, cron, and landing page.
• Adds production deployment docs, Meta App Review notes, Vercel cron config, Railway worker entrypoint, and CI/build validation envs.
• Moves from deprecated middleware.ts to Next.js 16 proxy.ts and adds tests for billing, env validation, OAuth/token encryption, webhook parsing, and worker behavior.

Validation

• npm run typecheck
• npm run lint
• npm test
• npm run build with placeholder production env values

Closes #4.
Closes #5.