Credit AR for gift-certificate redemptions in balance recompute (audit O7)

ApplyGiftCertificate posts DR 2500 Gift Certificate Liability / CR AR, but the AR
recompute only subtracted payments and credit-memo applications — so the redemption's
2500 debit was recomputed while its AR credit was not, leaving the Trial Balance out
of balance by the total gift-certificate amount redeemed and overstating AR on the
Balance Sheet.

Subtract GC redemptions from AR in both recompute engines:
  - FinancialReportService: Balance Sheet (gcRedeemedBs) and Trial Balance (gcRedeemedTb)
  - LedgerService: AR section (dated rows) and ComputePriorBalanceAsync (prior balance)

AR Aging was already correct (uses BalanceDue, which includes GiftCertificateRedeemed).
Adds a LedgerService regression test. Build clean; 292 unit tests pass.

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

tests/PowderCoating.UnitTests/LedgerServiceTests.cs

@@ -619,6 +619,42 @@ public class LedgerServiceTests
         Assert.Equal(-150m, ledger.ClosingBalance);
     }
 
+    // ── Gift-certificate redemptions credit AR (DR 2500 / CR AR) — must appear in the AR ──
+    // ── recompute or the trial balance is left out of balance by the redeemed amount (O7). ──
+
+    [Fact]
+    public async Task GetAccountLedgerAsync_AR_GiftCertificateRedemption_CreditsAccountsReceivable()
+    {
+        await using var context = CreateContext();
+        SeedAccount(context, id: 1, AccountSubType.AccountsReceivable);
+        SeedCustomer(context, id: 1);
+        context.Invoices.Add(new Invoice
+        {
+            Id = 10, CompanyId = 1, InvoiceNumber = "INV-GC", CustomerId = 1,
+            Status = InvoiceStatus.Sent, InvoiceDate = InPeriod, Total = 200m
+        });
+        context.GiftCertificates.Add(new GiftCertificate
+        {
+            Id = 5, CompanyId = 1, CertificateCode = "GC-5", OriginalAmount = 50m, RedeemedAmount = 50m,
+            IssueDate = InPeriod, Status = GiftCertificateStatus.FullyRedeemed
+        });
+        context.GiftCertificateRedemptions.Add(new GiftCertificateRedemption
+        {
+            Id = 1, CompanyId = 1, GiftCertificateId = 5, InvoiceId = 10,
+            AmountRedeemed = 50m, RedeemedDate = InPeriod
+        });
+        await context.SaveChangesAsync();
+
+        var ledger = await CreateService(context).GetAccountLedgerAsync(1, PeriodStart, PeriodEnd);
+
+        var invoiceEntry = ledger!.Entries.Single(e => e.Source == "Invoice");
+        var gcEntry      = ledger.Entries.Single(e => e.Source == "Gift Certificate");
+        Assert.Equal(200m, invoiceEntry.Debit);
+        Assert.Equal(50m, gcEntry.Credit);
+        Assert.Equal(0m, gcEntry.Debit);
+        Assert.Equal(150m, ledger.ClosingBalance); // debit-normal AR: 200 invoiced − 50 redeemed
+    }
+
     private static LedgerService CreateService(ApplicationDbContext context)
         => new LedgerService(context, Mock.Of<ILogger<LedgerService>>());