docs: add ARCHITECTURE.md for new contributors

[paruuup]

Summary

Changes

• Created docs/ARCHITECTURE.md with a Mermaid system flow diagram covering Next.js frontend, OpenAI, Supabase PostgreSQL, Clerk Auth, and Razorpay
• Added short explanation of each layer
• Added key files reference for new contributors

Test plan

• pnpm lint
• pnpm typecheck
• pnpm build
• Tested in browser (describe flows):

Checklist

• Linked to an existing issue (or explained why not needed)
• Did not commit secrets or .env.local
• Updated README / env docs if configuration changed
• Reused existing services where possible (no duplicate payment/auth logic)
• Verified API response shapes still match frontend callers

[vercel]

@priyankaganvatkar is attempting to deploy a commit to the notsoocool's projects Team on Vercel.

A member of the Team first needs to authorize it.

[paruuup]

Hi @notsoocool, I have submitted a PR for this issue.PR link:396c187