Sets (or overrides) an employee's Business Impact classification, mirroring ReportingController.UpdateBusinessImpact. Loads the tenant's risk scoring data, locates the employee by email (404 if not found), records the new BICategory, recomputes the Business Impact score from the category (Critical: humanRiskScore * 0.4, High: humanRiskScore * 0.2, Moderate: 0, Low: -abs(humanRiskScore * 0.2)), marks the classification as manually fixed, and adjusts the employee's human risk score by the delta between the new and previous Business Impact score (capped at 100).