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/SeedDataService.Accounts.cs

@@ -77,6 +77,7 @@ public partial class SeedDataService
             // A credit-normal account with a debit balance appears in the Trial Balance debit column,
             // reducing net revenue to match the discounted AR amount that was posted.
             new Account { AccountNumber = "4950", Name = "Sales Discounts",           AccountType = AccountType.Revenue,    AccountSubType = AccountSubType.OtherIncome,         IsSystem = true,  IsActive = true, Description = "Contra-revenue for invoice discounts granted to customers", CompanyId = company.Id, CreatedAt = now },
+            new Account { AccountNumber = "4960", Name = "Sales Returns & Allowances", AccountType = AccountType.Revenue,    AccountSubType = AccountSubType.OtherIncome,         IsSystem = true,  IsActive = true, Description = "Contra-revenue for the revenue portion of customer refunds (reversed sales)", CompanyId = company.Id, CreatedAt = now },
 
             // ── COST OF GOODS SOLD ────────────────────────────────────────────
             new Account { AccountNumber = "5000", Name = "Cost of Goods Sold",        AccountType = AccountType.CostOfGoods, AccountSubType = AccountSubType.CostOfGoodsSold,   IsSystem = false, IsActive = true, Description = "Direct cost of services delivered",          CompanyId = company.Id, CreatedAt = now },
@@ -141,6 +142,30 @@ public partial class SeedDataService
             added++;
         }
 
+        // 4960 Sales Returns & Allowances — contra-revenue account that receives the revenue portion
+        // of customer refunds under the "reverse the sale" model (DR Sales Returns + DR Sales Tax / CR Bank).
+        var has4960 = await _context.Set<Account>()
+            .IgnoreQueryFilters()
+            .AnyAsync(a => a.CompanyId == company.Id && a.AccountNumber == "4960" && !a.IsDeleted);
+
+        if (!has4960)
+        {
+            _context.Set<Account>().Add(new Account
+            {
+                AccountNumber  = "4960",
+                Name           = "Sales Returns & Allowances",
+                AccountType    = AccountType.Revenue,
+                AccountSubType = AccountSubType.OtherIncome,
+                IsSystem       = true,
+                IsActive       = true,
+                Description    = "Contra-revenue for the revenue portion of customer refunds (reversed sales)",
+                CompanyId      = company.Id,
+                CreatedAt      = now
+            });
+            await _context.SaveChangesAsync();
+            added++;
+        }
+
         return added;
     }
 }