feat: split bounty payouts across PR contributors

[ryanll]

Summary

• derive payout recipients from the merged PR author plus all paginated PR commit authors/committers and co-author trailers
• split the bounty amount evenly by cents across unique contributors
• persist payout and activity events per recipient while keeping the existing single-recipient response fields compatible
• show a split-payout success message in the maintainer approval UI

Fixes #12.

Notes

This preserves current single-recipient behavior when only one contributor is found. For split payouts, the primary PR author still supports PR-body wallet extraction; additional contributors resolve through the connected GitHub account/email path or fall back to the existing unclaimed payout notification.

Unlike implementations that only fetch the first page of PR commits, this uses Octokit pagination so contributors are not missed on larger PRs.

Verification

• ./node_modules/.bin/tsc --noEmit
• ./node_modules/.bin/eslint 'app/b/[owner]/[repo]/issues/[issueNumber]/approve-button.tsx' lib/api/client.ts lib/bounty/services/approve-payout.ts lib/bounty/payout-recipients.ts
• git diff --cached --check

[vercel]

@ryanll is attempting to deploy a commit to the skndash96's projects Team on Vercel.

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[bountic-bot]

⚡️ Bounty Competition Started

@ryanll has submitted a PR that references an issue with a bounty. Learn more about this bounty: https://bountic.vercel.app/b/skndash96/bountic/issues/12

When this PR is merged, the bounty will be locked and ready for payout approval.

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Bountic: Autonomous USDC bounties for open source