Implement core KPI calculation logic (OTD, Fill Rate, Lead Time, Inventory Turnover)

Develop SQL/model logic to compute key KPIs from loaded data in the database. Provide tests and doc for each metric. KPIs:
- On-Time Delivery (OTD)
- Fill Rate
- Lead Time (order to delivery)
- Inventory Turnover

Tasks:
- [ ] SQL/model functions for each KPI with clear input & output
- [ ] Backtest with seed/historical data
- [ ] Unit/integration tests for calculations
- [ ] Document KPI definitions, formulas, business rules (e.g. late order defined how?)

Deliverable: Working KPI logic with test coverage + sample result.

[EXPORT_TMC_SPL.XLSX](https://github.com/user-attachments/files/24993354/EXPORT_TMC_SPL.XLSX)