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>

src/PowderCoating.Infrastructure/Services/LedgerService.cs

@@ -2,6 +2,7 @@ using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
 using PowderCoating.Application.DTOs.Accounting;
 using PowderCoating.Application.Interfaces;
+using PowderCoating.Core.Accounting;
 using PowderCoating.Core.Enums;
 using PowderCoating.Infrastructure.Data;
 
@@ -199,6 +200,43 @@ public class LedgerService : ILedgerService
                 LinkId = inv.Id
             });
 
+        // ── 5b. Cash refunds reverse the sale (DR Sales Returns 4960 + DR Sales Tax Payable) ──
+        // The revenue portion debits Sales Returns; the tax portion debits the invoice's sales-tax
+        // account (relieving the liability). Cash leaving the bank is handled in the bank section above.
+        // Store-credit refunds are excluded — they post via CreditMemo, not the GL (see CancelRefund).
+        if (account.AccountNumber == "4960" || account.AccountType == AccountType.Liability)
+        {
+            var saleReversingRefunds = await _context.Refunds
+                .Include(r => r.Invoice)
+                .Where(r => !r.IsDeleted && r.Invoice != null
+                         && r.RefundMethod != PaymentMethod.StoreCredit
+                         && r.RefundDate >= fromDate && r.RefundDate <= toDate)
+                .ToListAsync();
+
+            foreach (var r in saleReversingRefunds)
+            {
+                var (returnsPortion, taxPortion) = RefundAllocation.Split(r.Amount, r.Invoice.TaxAmount, r.Invoice.Total);
+
+                if (account.AccountNumber == "4960" && returnsPortion != 0)
+                    entries.Add(new LedgerEntryDto
+                    {
+                        Date = r.RefundDate, Reference = r.Reference ?? $"REF-{r.Id}", Source = "Refund",
+                        Description = $"Sales return — {r.Invoice.InvoiceNumber}",
+                        Debit = returnsPortion, Credit = 0,
+                        LinkController = "Invoices", LinkId = r.InvoiceId
+                    });
+
+                if (r.Invoice.SalesTaxAccountId == accountId && taxPortion != 0)
+                    entries.Add(new LedgerEntryDto
+                    {
+                        Date = r.RefundDate, Reference = r.Reference ?? $"REF-{r.Id}", Source = "Refund",
+                        Description = $"Tax refunded — {r.Invoice.InvoiceNumber}",
+                        Debit = taxPortion, Credit = 0,
+                        LinkController = "Invoices", LinkId = r.InvoiceId
+                    });
+            }
+        }
+
         // ── 6. Direct expenses categorized to this account (DEBIT) ────────────
         // e.g. Expense account 6200 receives direct expense entries
         var expensesTo = await _context.Expenses
@@ -312,24 +350,8 @@ public class LedgerService : ILedgerService
                     LinkId = cm.InvoiceId
                 });
 
-            // Refunds re-open AR (DEBIT — customer owes again after refund)
-            var arRefunds = await _context.Refunds
-                .Include(r => r.Invoice)
-                .Where(r => r.RefundDate >= fromDate && r.RefundDate <= toDate && !r.IsDeleted)
-                .ToListAsync();
-
-            foreach (var r in arRefunds)
-                entries.Add(new LedgerEntryDto
-                {
-                    Date = r.RefundDate,
-                    Reference = r.Reference ?? $"REF-{r.Id}",
-                    Source = "Refund",
-                    Description = r.Reason,
-                    Debit = r.Amount,
-                    Credit = 0,
-                    LinkController = "Invoices",
-                    LinkId = r.InvoiceId
-                });
+            // NOTE: cash refunds no longer touch AR. Under the "reverse the sale" model they debit
+            // Sales Returns + Sales Tax Payable and credit the bank (see section 5b above).
         }
 
         // ── 9. Accounts Payable ────────────────────────────────────────────────
@@ -594,6 +616,27 @@ public class LedgerService : ILedgerService
                      && i.InvoiceDate < beforeDate)
             .SumAsync(i => (decimal?)i.TaxAmount) ?? 0;
 
+        // 5b. Cash refunds reverse the sale (DR Sales Returns 4960 + DR Sales Tax Payable). Store-credit
+        // refunds are excluded (no GL posting). Mirrors section 5b in GetAccountLedgerAsync.
+        if (account.AccountNumber == "4960" || account.AccountType == AccountType.Liability)
+        {
+            var priorRefunds = await _context.Refunds
+                .Include(r => r.Invoice)
+                .Where(r => !r.IsDeleted && r.Invoice != null
+                         && r.RefundMethod != PaymentMethod.StoreCredit
+                         && r.RefundDate < beforeDate)
+                .ToListAsync();
+
+            foreach (var r in priorRefunds)
+            {
+                var (returnsPortion, taxPortion) = RefundAllocation.Split(r.Amount, r.Invoice.TaxAmount, r.Invoice.Total);
+                if (account.AccountNumber == "4960")
+                    debits += returnsPortion;
+                if (r.Invoice.SalesTaxAccountId == accountId)
+                    debits += taxPortion;
+            }
+        }
+
         // 6. Direct expenses categorized to this account (DEBIT)
         debits += await _context.Expenses
             .Where(e => e.ExpenseAccountId == accountId && e.Date < beforeDate)
@@ -624,9 +667,8 @@ public class LedgerService : ILedgerService
                 .Where(a => a.AppliedDate < beforeDate && a.Invoice.Status != InvoiceStatus.Voided)
                 .SumAsync(a => (decimal?)a.AmountApplied) ?? 0;
 
-            debits += await _context.Refunds
-                .Where(r => !r.IsDeleted && r.RefundDate < beforeDate)
-                .SumAsync(r => (decimal?)r.Amount) ?? 0;
+            // NOTE: cash refunds no longer debit AR — they reverse the sale (Sales Returns + Sales Tax),
+            // handled in section 5b above.
         }
 
         // 9. Accounts Payable