feat(billing): model invoice payment balances and settlement

Persist amounts due and paid, derive partial and full settlement, and protect balances during command updates. Add migration, focused tests and invoicing documentation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/Yavsc.Abstract/Billing/IBillable.cs

@@ -20,6 +20,8 @@ namespace Yavsc.Billing
 
         DateTime? ValidationDate { get; }
 
+        PaymentSummary? PaymentSummary { get; }
+
         bool GetIsAcquitted ();
 
         string GetFileBaseName (IBillingService service);

src/Yavsc.Abstract/Billing/PaymentStatus.cs

@@ -0,0 +1,8 @@
+namespace Yavsc.Billing;
+
+public enum PaymentStatus
+{
+    Unpaid,
+    PartiallyPaid,
+    Paid
+}

src/Yavsc.Abstract/Billing/PaymentSummary.cs

@@ -0,0 +1,21 @@
+namespace Yavsc.Billing;
+
+public sealed class PaymentSummary
+{
+    public PaymentSummary(decimal amountDue, decimal amountPaid)
+    {
+        ArgumentOutOfRangeException.ThrowIfNegative(amountDue);
+        ArgumentOutOfRangeException.ThrowIfNegative(amountPaid);
+
+        AmountDue = amountDue;
+        AmountPaid = amountPaid;
+    }
+
+    public decimal AmountDue { get; }
+    public decimal AmountPaid { get; }
+    public decimal AmountRemaining => Math.Max(0m, AmountDue - AmountPaid);
+    public bool IsAcquitted => AmountPaid >= AmountDue;
+    public PaymentStatus Status => IsAcquitted
+        ? PaymentStatus.Paid
+        : AmountPaid > 0m ? PaymentStatus.PartiallyPaid : PaymentStatus.Unpaid;
+}