Make cash refunds reverse the sale instead of re-opening AR

Audit finding 9a: a cash refund posted DR AR (control up) while the customer
subledger went down — opposite directions, guaranteeing AR drift. Per the chosen
model, a cash refund now *reverses the sale*: DR Sales Returns (revenue portion)
+ DR Sales Tax Payable (tax portion) / CR Bank, with the customer AR balance left
untouched (the invoice stays paid; the sale is contra'd).

- New 4960 "Sales Returns & Allowances" contra-revenue account (seed + self-heal).
- Tax/revenue split centralized in Core RefundAllocation.Split so the posting and
  both reporting recomputes compute it identically (a mismatch would unbalance the
  trial balance).
- InvoicesController IssueRefund/CancelRefund: new posting + reversal; no AR/
  customer-balance change.
- LedgerService (per-account + prior balance): refunds debit Sales Returns + Sales
  Tax, no longer debit AR; bank credit unchanged.
- FinancialReportService: trial balance, balance sheet (retained earnings + tax
  liability), and P&L (contra-revenue line) all updated to match. Store-credit
  refunds are excluded everywhere (they post via CreditMemo, not the GL — 9b).

Verified: $108 refund (incl. $8 tax) -> bank -100... checks: assets -108 =
liabilities -8 + equity -100. New tests cover the split invariant and the ledger
postings. Build clean; 283 unit tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/PowderCoating.UnitTests/LedgerServiceTests.cs

@@ -509,6 +509,57 @@ public class LedgerServiceTests
 
     // ── Helpers ───────────────────────────────────────────────────────────
 
+    // ── Cash refund reverses the sale: Sales Returns + Sales Tax debited, AR untouched ──
+
+    [Fact]
+    public async Task GetAccountLedgerAsync_CashRefund_ReversesTheSale_DebitsReturnsAndTax_NotAr()
+    {
+        await using var context = CreateContext();
+
+        context.Accounts.Add(new Account { Id = 1, CompanyId = 1, AccountNumber = "1000", Name = "Checking",          AccountType = AccountType.Asset,     AccountSubType = AccountSubType.Checking,             IsActive = true });
+        context.Accounts.Add(new Account { Id = 2, CompanyId = 1, AccountNumber = "1100", Name = "AR",                AccountType = AccountType.Asset,     AccountSubType = AccountSubType.AccountsReceivable,   IsActive = true });
+        context.Accounts.Add(new Account { Id = 3, CompanyId = 1, AccountNumber = "2200", Name = "Sales Tax Payable", AccountType = AccountType.Liability, AccountSubType = AccountSubType.OtherCurrentLiability, IsActive = true });
+        context.Accounts.Add(new Account { Id = 4, CompanyId = 1, AccountNumber = "4960", Name = "Sales Returns",     AccountType = AccountType.Revenue,   AccountSubType = AccountSubType.OtherIncome,          IsActive = true });
+
+        // Invoice $108 incl. $8 tax, tax posted to account 3.
+        context.Invoices.Add(new Invoice
+        {
+            Id = 99, CompanyId = 1, InvoiceNumber = "INV-0099", CustomerId = 1,
+            Status = InvoiceStatus.Sent, InvoiceDate = InPeriod,
+            Total = 108m, TaxAmount = 8m, SalesTaxAccountId = 3
+        });
+
+        // Full cash refund of $108 paid from Checking.
+        context.Refunds.Add(new Refund
+        {
+            Id = 1, CompanyId = 1, InvoiceId = 99, Amount = 108m,
+            RefundDate = InPeriod, RefundMethod = PaymentMethod.Check, DepositAccountId = 1
+        });
+        await context.SaveChangesAsync();
+
+        var svc = CreateService(context);
+
+        // Sales Returns (4960): revenue portion debited ($100).
+        var returns = await svc.GetAccountLedgerAsync(4, PeriodStart, PeriodEnd);
+        var returnsEntry = Assert.Single(returns!.Entries, e => e.Source == "Refund");
+        Assert.Equal(100m, returnsEntry.Debit);
+        Assert.Equal(0m, returnsEntry.Credit);
+
+        // Sales Tax Payable: tax portion debited ($8), relieving the liability.
+        var tax = await svc.GetAccountLedgerAsync(3, PeriodStart, PeriodEnd);
+        var taxEntry = Assert.Single(tax!.Entries, e => e.Source == "Refund");
+        Assert.Equal(8m, taxEntry.Debit);
+
+        // Bank: full $108 credited (cash leaves).
+        var bank = await svc.GetAccountLedgerAsync(1, PeriodStart, PeriodEnd);
+        var bankEntry = Assert.Single(bank!.Entries, e => e.Source == "Refund");
+        Assert.Equal(108m, bankEntry.Credit);
+
+        // AR is no longer touched by refunds under the "reverse the sale" model.
+        var ar = await svc.GetAccountLedgerAsync(2, PeriodStart, PeriodEnd);
+        Assert.DoesNotContain(ar!.Entries, e => e.Source == "Refund");
+    }
+
     private static LedgerService CreateService(ApplicationDbContext context)
         => new LedgerService(context, Mock.Of<ILogger<LedgerService>>());