docs: rewrite README for the current bounty marketplace

[arturstar992-eng]

Summary

• replace the obsolete contribution/NFT/SOL description with the current GitHub bounty marketplace workflow
• document the actual Next.js 15, Privy, Prisma/PostgreSQL, Octokit, and Solana stack
• add accurate local setup, environment variables, webhook configuration, API reference, project structure, and security guidance
• remove broken clone instructions, mojibake headings, and references to files or features that are not present
• call out the current MVP/archived-code boundary so contributors know which implementation is active

Verification

• cross-checked the guide against package.json, prisma/schema.prisma, the active dashboard, authentication helpers, and API routes
• verified all relative README links resolve to files in the repository
• ran git diff --check

Closes #40

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