feat: multi-contributor bounty payout distribution

[hanwucui]

Summary

Parses Co-authored-by: trailers from merged PRs and splits bounty payouts equally among all contributors.

Changes

• New lib/bounty/coauthors.ts — parse co-author trailers from PR body, extract GitHub usernames from noreply emails, deduplicate
• DB migration — adds winning_pr_coauthors jsonb column to �ounties table
• PR merge handler — extracts and stores co-authors when PR is merged
• Payout service — new
  esolveAndPayoutMulti() for equal split (remainder to PR author)
• Approve payout — branches on single vs multi-contributor; creates separate payout/activity events per contributor
• Ledger comment — shows all contributors and per-person split in GitHub locked comment
• UI — bounty detail page shows co-authors and split info; activity feed shows split amounts
• Types — BountyDetail and database types updated

Design Decisions

• Backward compatible — single-author PRs follow the exact same code path
• Equal split with rounding remainder to PR author
• GitHub username required for co-authors (derived from noreply addresses)
• Per-contributor audit trail via separate payout_events and activity_events rows

Closes #12

[vercel]

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[bountic-bot]

⚡️ Bounty Competition Started

@hanwucui 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