Backend:
- Added `SettlementActivity` model to track payments against specific expense shares.
- Added `status` and `paid_at` to `ExpenseSplit` model.
- Added `overall_settlement_status` to `Expense` model.
- Implemented CRUD for `SettlementActivity`, including logic to update parent expense/split statuses.
- Updated `Expense` CRUD to initialize new status fields.
- Defined Pydantic schemas for `SettlementActivity` and updated `Expense/ExpenseSplit` schemas.
- Exposed API endpoints for creating/listing settlement activities and settling shares.
- Adjusted group balance summary logic to include settlement activities.
- Added comprehensive backend unit and API tests for new functionality.
Frontend (Foundation & TODOs due to my current capabilities):
- Created TypeScript interfaces for all new/updated models.
- Set up `listDetailStore.ts` with an action to handle `settleExpenseSplit` (API call is a placeholder) and refresh data.
- Created `SettleShareModal.vue` component for payment confirmation.
- Added unit tests for the new modal and store logic.
- Updated `ListDetailPage.vue` to display detailed expense/share statuses and settlement activities.
- `mitlist_doc.md` updated to reflect all backend changes and current frontend status.
- A `TODO.md` (implicitly within `mitlist_doc.md`'s new section) outlines necessary manual frontend integrations for `api.ts` and `ListDetailPage.vue` to complete the 'Settle Share' UI flow.
This set of changes provides the core backend infrastructure for precise expense share tracking and settlement, and lays the groundwork for full frontend integration.
