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>

src/PowderCoating.Infrastructure/Services/LedgerService.cs

@@ -495,6 +495,39 @@ public class LedgerService : ILedgerService
                 });
         }
 
+        // ── 12b. Customer Credits liability (account 2350) ────────────────────
+        // CR when a credit memo (incl. store-credit refund) is issued; DR when applied to an invoice.
+        // Voided memos are excluded (their issue/void net to zero).
+        if (account.AccountNumber == "2350")
+        {
+            var memosIssued = await _context.CreditMemos
+                .Where(m => m.Status != CreditMemoStatus.Voided
+                         && m.IssueDate >= fromDate && m.IssueDate <= toDate)
+                .ToListAsync();
+            foreach (var m in memosIssued)
+                entries.Add(new LedgerEntryDto
+                {
+                    Date = m.IssueDate, Reference = m.MemoNumber,
+                    Source = "Credit Memo", Description = "Store credit issued",
+                    Debit = 0, Credit = m.Amount,
+                    LinkController = "CreditMemos", LinkId = m.Id
+                });
+
+            var memosApplied = await _context.CreditMemoApplications
+                .Include(a => a.CreditMemo).Include(a => a.Invoice)
+                .Where(a => a.CreditMemo.Status != CreditMemoStatus.Voided
+                         && a.AppliedDate >= fromDate && a.AppliedDate <= toDate)
+                .ToListAsync();
+            foreach (var a in memosApplied)
+                entries.Add(new LedgerEntryDto
+                {
+                    Date = a.AppliedDate, Reference = a.CreditMemo?.MemoNumber ?? $"CM-{a.CreditMemoId}",
+                    Source = "Credit Applied", Description = $"Applied to {a.Invoice?.InvoiceNumber}",
+                    Debit = a.AmountApplied, Credit = 0,
+                    LinkController = "Invoices", LinkId = a.InvoiceId
+                });
+        }
+
         // ── 10. Journal Entry lines touching this account ──────────────────
         var jeLines = await _context.JournalEntryLines
             .Include(l => l.JournalEntry)
@@ -716,6 +749,17 @@ public class LedgerService : ILedgerService
                 .SumAsync(d => (decimal?)d.Amount) ?? 0;
         }
 
+        // 12b. Customer Credits liability (account 2350)
+        if (account.AccountNumber == "2350")
+        {
+            credits += await _context.CreditMemos
+                .Where(m => m.Status != CreditMemoStatus.Voided && m.IssueDate < beforeDate)
+                .SumAsync(m => (decimal?)m.Amount) ?? 0;
+            debits += await _context.CreditMemoApplications
+                .Where(a => a.CreditMemo.Status != CreditMemoStatus.Voided && a.AppliedDate < beforeDate)
+                .SumAsync(a => (decimal?)a.AmountApplied) ?? 0;
+        }
+
         // 10. Posted journal entry lines touching this account (prior to period)
         debits += await _context.JournalEntryLines
             .Where(l => l.AccountId == accountId