docs: refresh bounty marketplace README

[Choonsiks]

Summary

• Rewrites the README around the current bounty marketplace MVP instead of the older contribution/NFT badge positioning.
• Documents sponsor and contributor flows, dashboard tabs, API routes, data models, environment variables, webhook setup, and security notes.
• Points readers to PRIVY_SETUP.md and MIGRATION_SUMMARY.md for deeper setup/migration context.

Verification

• git diff --check
• Basic README sanity check: balanced fenced code blocks and current-stack terms verified.

Closes #40

[vercel]

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