refactor(billing): adopt Money value object across Wallet, Invoice and BillingPlan

src/BuildingBlocks/Core/Domain/Money.cs

@@ -13,4 +13,50 @@ public Money(decimal amount, string currency)
     }
 
     public static Money Zero(string currency = "USD") => new(0m, currency);
+
+    public Money Add(Money other)
+    {
+        ArgumentNullException.ThrowIfNull(other);
+        EnsureSameCurrency(other);
+        return this with { Amount = Amount + other.Amount };
+    }
+
+    public Money Subtract(Money other)
+    {
+        ArgumentNullException.ThrowIfNull(other);
+        EnsureSameCurrency(other);
+        return this with { Amount = Amount - other.Amount };
+    }
+
+    public Money Multiply(decimal factor) => this with { Amount = Amount * factor };
+
+    public Money Round(int decimals) =>
+        this with { Amount = Math.Round(Amount, decimals, MidpointRounding.AwayFromZero) };
+
+    public static Money operator +(Money left, Money right)
+    {
+        ArgumentNullException.ThrowIfNull(left);
+        return left.Add(right);
+    }
+
+    public static Money operator -(Money left, Money right)
+    {
+        ArgumentNullException.ThrowIfNull(left);
+        return left.Subtract(right);
+    }
+
+    public static Money operator *(Money left, decimal right)
+    {
+        ArgumentNullException.ThrowIfNull(left);
+        return left.Multiply(right);
+    }
+
+    private void EnsureSameCurrency(Money other)
+    {
+        if (!string.Equals(Currency, other.Currency, StringComparison.Ordinal))
+        {
+            throw new InvalidOperationException(
+                $"Cannot operate on Money with different currencies: {Currency} and {other.Currency}.");
+        }
+    }
 }

src/Host/FSH.Starter.DbMigrator/DemoSeed/DemoSeeder.cs

@@ -233,7 +233,7 @@ private async Task SeedTenantSubscriptionAsync(DemoTenant demo, CancellationToke
 
         // Paid plans get an issued term invoice (like real CreateTenant), written directly so no InvoiceIssuedIntegrationEvent fires
         // (no outbox dispatcher; seeding mustn't email). Idempotent on invoice number; free plans (term price 0) get none, as in production.
-        if (plan.TermPrice > 0m)
+        if (plan.TermPrice.Amount > 0m)
         {
             var invoiceNumber = string.Create(
                 CultureInfo.InvariantCulture, $"SUB-{startUtc:yyyyMM}-{demo.Id.ToUpperInvariant()}");
@@ -251,7 +251,7 @@ private async Task SeedTenantSubscriptionAsync(DemoTenant demo, CancellationToke
                         CultureInfo.InvariantCulture,
                         $"{plan.Name} — {plan.Interval} subscription ({startUtc:yyyy-MM-dd} to {endUtc:yyyy-MM-dd})"),
                     1m,
-                    plan.TermPrice);
+                    plan.TermPrice.Amount);
                 invoice.Issue();
                 billingDb.Invoices.Add(invoice);
                 if (_logger.IsEnabled(LogLevel.Information))