docs: refresh README for current bounty workflow

[TamerMansour-AI]

Summary

• Rewrites the README around the current USDC GitHub bounty marketplace instead of the older SOL/NFT reputation product.
• Documents the actual stack: Next.js 15, React 19, Prisma, PostgreSQL, Privy, Octokit, GitHub webhooks, and Vercel.
• Adds practical sections for product flow, repository structure, data models, API routes, environment variables, local development, webhook setup, and current placeholder/legacy notes.
• Removes inaccurate references to NextAuth, .env.example, SOL transaction prerequisites, automatic NFT minting, and the missing MIT license file.

Validation

• Reviewed package.json, prisma/schema.prisma, PRIVY_SETUP.md, API route structure, and GitHub webhook code before updating the docs.
• Ran git diff --check successfully.

Notes

This is intentionally scoped to README.md for issue #40.

[vercel]

@TamerMansour-AI is attempting to deploy a commit to the Andrei's projects Team on Vercel.

A member of the Team first needs to authorize it.

[TamerMansour-AI]

Small status note for review: the only failing check is Vercel preview authorization for the maintainer team, not a code/test failure from this README-only change. The branch is mergeable and GitGuardian passed with no secrets detected.