Get Monthly Sales

Get total monthly sales for the authenticated technician.

Returns the sum of all estimate subtotals sold by the technician for the specified month, plus the potential sales from all active followups (only for current month).

Args: month: Optional month in YYYY-MM format. If not provided, defaults to current month.

Returns: { "total_sales": 12500.50, "estimate_count": 15, "followup_sales": 3200.00, "followup_sales_count": 8, "potential_followup_sales": 5000.00 // Only included for current month }