feat: Multiple PR contributors payout distribution with weighted splits

[watcharaponthod-code]

Resolves #12.

This PR implements the requested feature to allow payout distribution across multiple contributors for a single bounty.

Key Features:

• Weighted Split Support: Added support for a new PR body tag <!-- bountic-split: @user1:weight, @user2:weight --> which allows maintainers/contributors to define custom distribution ratios.
• Automatic Fallback: If no split tag is present, the payout defaults to the primary PR author (backward compatible).
• Atomic Distribution: Payouts are processed for all recipients during the approval flow, with individual records created in payout_events and activity_events.
• Precision Math: Implemented integer-cent math to ensure the total bounty is distributed accurately without rounding errors.

Note on Issue #12 Description:

I noticed that the body of issue #12 mentioned an inventory_quantity bug in a cart GET endpoint. This implementation focuses on the actual feature request in the title: "Multiple PR contributors payout distribution", which is also listed as a future goal in PLAN.md.

Verification:

• Verified the split parsing logic with various inputs.
• Verified the cent-distribution math to ensure total consistency.

[vercel]

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

A member of the Team first needs to authorize it.

[bountic-bot]

⚡️ Bounty Competition Started

@watcharaponthod-code 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