docs: align README with bounty marketplace implementation

[iota3-cell]

Summary

• Rewrites the README around the current GitHub bounty marketplace implementation instead of the old SOL/NFT badge positioning.
• Documents the active product workflow, stack, repository layout, API routes, Prisma models, webhook behavior, and current limitations.
• Calls out unfinished payment settlement and GitHub label automation so new contributors do not rely on outdated claims.

Checks

• npx prettier --check README.md
• pnpm lint was attempted through the repository pre-commit hook, but local pnpm 11 blocked install because build-script approvals are not configured in this checkout. No application code was changed.

Closes #40

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