Track store credit as a Customer Credits liability (GL)

Audit finding 9b: store-credit refunds and credit memos posted nothing to the GL
on issue (only a CreditMemo + Customer.CreditBalance), so outstanding store credit
was invisible on the balance sheet and the contra-revenue was recognized only on
apply. Introduces a 2350 "Customer Credits" liability so the credit is on the books
from issue to apply.

Model (chosen): lifecycle-equivalent to before, plus the liability is tracked.
- Issue (credit memos, goodwill, and store-credit refunds): DR Sales Discounts
  (4950) / CR Customer Credits (2350).
- Apply: DR Customer Credits / CR AR (was DR Sales Discounts / CR AR).
- Void unapplied remainder: DR Customer Credits / CR Sales Discounts.

Posting updated in all 8 sites: CreditMemosController Create/Apply/Void and
InvoicesController IssueCreditMemo/IssueRefund(store credit)/ApplyCredit/
VoidCreditMemo/CancelRefund. New 2350 account (seed + self-heal).

Reporting moved in lockstep so the books still balance: the 4950 contra-revenue
shifts from applied -> issued (active memos in full + applied portion of voided),
the 2350 liability = unapplied balance on active memos, AR still credited by
applications. Updated in FinancialReportService (balance sheet retained earnings,
trial balance, P&L) and LedgerService (per-account + prior-balance 2350 section).
Verified the balance-sheet identity for active and voided memos by hand; new
ledger test covers the 2350 lifecycle. Build clean; 284 unit tests pass.

Note: pre-existing quirks left untouched (out of 9b scope) — account 2300 is
seeded as "Payroll Liabilities" but resolved as Customer Deposits in code, and
LedgerService doesn't recompute 4950 so RecalculateBalances understates it; both
predate this change.

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

tests/PowderCoating.UnitTests/LedgerServiceTests.cs

@@ -560,6 +560,31 @@ public class LedgerServiceTests
         Assert.DoesNotContain(ar!.Entries, e => e.Source == "Refund");
     }
 
+    // ── Credit memo: Customer Credits (2350) tracks outstanding store credit, excludes voided ──
+
+    [Fact]
+    public async Task GetAccountLedgerAsync_CustomerCredits2350_TracksOutstandingStoreCredit_ExcludesVoided()
+    {
+        await using var context = CreateContext();
+
+        context.Accounts.Add(new Account { Id = 1, CompanyId = 1, AccountNumber = "2350", Name = "Customer Credits", AccountType = AccountType.Liability, AccountSubType = AccountSubType.OtherCurrentLiability, IsActive = true });
+        SeedInvoice(context, id: 99);
+
+        // Active memo: $100 issued, $40 applied → $60 outstanding liability.
+        context.CreditMemos.Add(new CreditMemo { Id = 1, CompanyId = 1, MemoNumber = "CM-1", CustomerId = 1, Amount = 100m, AmountApplied = 40m, IssueDate = InPeriod, Status = CreditMemoStatus.PartiallyApplied });
+        context.CreditMemoApplications.Add(new CreditMemoApplication { Id = 1, CompanyId = 1, CreditMemoId = 1, InvoiceId = 99, AmountApplied = 40m, AppliedDate = InPeriod });
+
+        // Voided memo: must contribute nothing to the liability.
+        context.CreditMemos.Add(new CreditMemo { Id = 2, CompanyId = 1, MemoNumber = "CM-2", CustomerId = 1, Amount = 50m, AmountApplied = 0m, IssueDate = InPeriod, Status = CreditMemoStatus.Voided });
+        await context.SaveChangesAsync();
+
+        var ledger = await CreateService(context).GetAccountLedgerAsync(1, PeriodStart, PeriodEnd);
+
+        // CR 100 (issue) + DR 40 (apply) on the active memo; the voided memo is excluded.
+        Assert.Equal(2, ledger!.Entries.Count);
+        Assert.Equal(60m, ledger.ClosingBalance); // credit-normal liability: 100 issued − 40 applied
+    }
+
     private static LedgerService CreateService(ApplicationDbContext context)
         => new LedgerService(context, Mock.Of<ILogger<LedgerService>>());