feat: implement period value calculation of the invoice

We are calculating the period value of the invoice based on the line items
min/max period values.

openmeter/billing/service/invoicecalc/calculator.go

@@ -10,6 +10,7 @@ import (
 var InvoiceCalculations = []Calculation{
 	DraftUntilIfMissing,
 	RecalculateDetailedLinesAndTotals,
+	CalculateInvoicePeriod,
 }
 
 type Calculation func(*billing.Invoice, CalculatorDependencies) error

openmeter/billing/service/invoicecalc/period.go

@@ -0,0 +1,36 @@
+package invoicecalc
+
+import "github.com/openmeterio/openmeter/openmeter/billing"
+
+// CalculateInvoicePeriod calculates the period of the invoice based on the lines.
+func CalculateInvoicePeriod(invoice *billing.Invoice, deps CalculatorDependencies) error {
+	var period *billing.Period
+
+	for _, line := range invoice.Lines.OrEmpty() {
+		if line.DeletedAt != nil || line.Status != billing.InvoiceLineStatusValid {
+			continue
+		}
+
+		if period == nil {
+			period = &billing.Period{
+				Start: line.Period.Start,
+				End:   line.Period.End,
+			}
+			continue
+		}
+
+		if line.Period.Start.Before(period.Start) {
+			period.Start = line.Period.Start
+		}
+
+		if line.Period.End.After(period.End) {
+			period.End = line.Period.End
+		}
+	}
+
+	if period != nil {
+		invoice.Period = period
+	}
+
+	return nil
+}