feat: split bounty payouts across PR contributors

[s3curi7y]

Resolves #12.

Summary

This adds multi-contributor payout support for merged bounty PRs. The payout approval flow now derives payout recipients from the primary PR author, commit authors, and Co-authored-by trailers, then splits the bounty amount across the resolved contributors.

Details

• Added a dedicated payout splitting helper with coverage for contributor deduping, GitHub noreply co-author emails, cent-accurate amount splitting, and single-author fallback.
• Approval now paginates PR commits before building the contributor set.
• Payout events are created as PENDING before external payout calls, then updated to SUCCESS or FAILED per recipient.
• Existing payout events block automatic retry so a maintainer can review partial or ambiguous payout state before any further action.
• Added a unique payout-event index for (issue_id, lower(recipient_username)) to guard against duplicate payout rows for the same recipient.
• Kept the existing single-recipient API fields for compatibility and added a payouts array for split payout details.

Validation

• corepack pnpm test
• corepack pnpm typecheck
• git diff --check

Notes

corepack pnpm lint currently reports pre-existing unrelated lint issues in files outside this change. corepack pnpm build compiles successfully but cannot complete prerendering without the required Supabase/App URL environment variables.

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[bountic-bot]

⚡️ Bounty Competition Started

@s3curi7y 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