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.Org/Migrations/20261019182518_InvoicePaymentAmounts.cs

@@ -0,0 +1,39 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Yavsc.Migrations
+{
+    /// <inheritdoc />
+    public partial class InvoicePaymentAmounts : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<decimal>(
+                name: "AmountDue",
+                table: "NominativeServiceCommands",
+                type: "numeric",
+                nullable: true);
+
+            migrationBuilder.AddColumn<decimal>(
+                name: "AmountPaid",
+                table: "NominativeServiceCommands",
+                type: "numeric",
+                nullable: false,
+                defaultValue: 0m);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "AmountDue",
+                table: "NominativeServiceCommands");
+
+            migrationBuilder.DropColumn(
+                name: "AmountPaid",
+                table: "NominativeServiceCommands");
+        }
+    }
+}

src/Yavsc.Org/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -17,7 +17,7 @@ namespace Yavsc.Migrations
         {
 #pragma warning disable 612, 618
             modelBuilder
-                .HasAnnotation("ProductVersion", "10.0.12")
+                .HasAnnotation("ProductVersion", "10.0.13")
                 .HasAnnotation("Relational:MaxIdentifierLength", 63);
 
             NpgsqlModelBuilderExtensions.UseIdentityByDefaultColumns(modelBuilder);
@@ -1454,6 +1454,14 @@ namespace Yavsc.Migrations
                         .IsRequired()
                         .HasColumnType("text");
 
+                    b.Property<decimal?>("AmountDue")
+                        .HasColumnType("numeric");
+
+                    b.Property<decimal>("AmountPaid")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("numeric")
+                        .HasDefaultValue(0m);
+
                     b.Property<string>("ClientId")
                         .IsRequired()
                         .HasColumnType("text");