Create or update a budget value for a project. Only available for projects with yearly budgets enabled. BudgetValue represents a project-level yearly aggregate identified by (year, exp_type). Use this endpoint to read or update CAPEX/OPEX yearly totals directly. For line-level budgeting per budget line and year, use BudgetLineValue endpoints. This endpoint behaves as upsert for (year, exp_type): returns 201 when created and 200 when updated.