Fix Customer Deposits account mislabel and Sales Discounts recalc (audit O1, O2)

O1: account 2300 has always been used by the deposit GL code as the Customer
Deposits liability (resolved by number), but it was seeded/named "Payroll
Liabilities" for tenants the AccountingDepositsGL migration's NOT EXISTS guard
skipped — so the liability was mislabeled on the balance sheet. Rename 2300 to
"Customer Deposits" (IsSystem) and move payroll to a new 2400 account:
  - both seed paths (SeedDataService.Accounts, SeedData)
  - EnsureSystemAccountsAsync self-heal (renames only where still default-named,
    preserving user renames; ensures 2400 exists)
  - migration RenameDepositsAccountAddPayroll for existing tenants
Account number 2300 is unchanged, so the deposit posting code needs no changes.

O2: LedgerService never recomputed 4950 Sales Discounts, so "Recalculate
Balances" wiped it to JE-only and the Balance Reconciliation report showed false
drift. Add a 4950 section to GetAccountLedgerAsync and ComputePriorBalanceAsync
that reproduces the actual postings (invoice discounts DR + credit-memo issuance
DR, less the unapplied remainder of voided memos CR), matching AccountBalanceService.

Adds a LedgerService regression test for 4950. Documents both fixes plus the
remaining open findings (O3, O4) in docs/ACCOUNTING_AUDIT.md so the audit is no
longer lost. Build clean; 291 unit tests pass; migration applied.

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

src/PowderCoating.Infrastructure/Services/LedgerService.cs

@@ -528,6 +528,57 @@ public class LedgerService : ILedgerService
                 });
         }
 
+        // ── 12c. Sales Discounts contra-revenue (account 4950) ────────────────
+        // Mirrors the actual postings made by AccountBalanceService so a balance recompute reproduces
+        // the stored CurrentBalance (otherwise "Recalculate Balances" would wipe 4950 down to JE-only):
+        //   • Invoice discounts        → DR 4950 at invoice date (InvoicesController invoice create/edit).
+        //   • Credit memo issuance     → DR 4950 = full memo amount at issue (CreditMemosController.Create
+        //     and the store-credit refund path, which both create a CreditMemo row).
+        //   • Credit memo void         → CR 4950 = unapplied remainder at void (reverses the unused part).
+        // Keep this in step with FinancialReportService's 4950 computation (discountsByAcct + cmContraRevenue).
+        if (account.AccountNumber == "4950")
+        {
+            var discountInvoices = await _context.Invoices
+                .Where(i => i.DiscountAmount > 0
+                         && i.Status != InvoiceStatus.Draft && i.Status != InvoiceStatus.Voided
+                         && i.InvoiceDate >= fromDate && i.InvoiceDate <= toDate)
+                .ToListAsync();
+            foreach (var inv in discountInvoices)
+                entries.Add(new LedgerEntryDto
+                {
+                    Date = inv.InvoiceDate, Reference = inv.InvoiceNumber,
+                    Source = "Invoice", Description = $"Discount on {inv.InvoiceNumber}",
+                    Debit = inv.DiscountAmount, Credit = 0,
+                    LinkController = "Invoices", LinkId = inv.Id
+                });
+
+            var discountMemosIssued = await _context.CreditMemos
+                .Where(m => m.IssueDate >= fromDate && m.IssueDate <= toDate)
+                .ToListAsync();
+            foreach (var m in discountMemosIssued)
+                entries.Add(new LedgerEntryDto
+                {
+                    Date = m.IssueDate, Reference = m.MemoNumber,
+                    Source = "Credit Memo", Description = "Store credit issued (contra-revenue)",
+                    Debit = m.Amount, Credit = 0,
+                    LinkController = "CreditMemos", LinkId = m.Id
+                });
+
+            var discountMemosVoided = await _context.CreditMemos
+                .Where(m => m.Status == CreditMemoStatus.Voided
+                         && m.UpdatedAt >= fromDate && m.UpdatedAt <= toDate
+                         && m.Amount > m.AmountApplied)
+                .ToListAsync();
+            foreach (var m in discountMemosVoided)
+                entries.Add(new LedgerEntryDto
+                {
+                    Date = m.UpdatedAt.GetValueOrDefault(), Reference = m.MemoNumber,
+                    Source = "Credit Memo Voided", Description = "Reversed unapplied store credit",
+                    Debit = 0, Credit = m.Amount - m.AmountApplied,
+                    LinkController = "CreditMemos", LinkId = m.Id
+                });
+        }
+
         // ── 10. Journal Entry lines touching this account ──────────────────
         var jeLines = await _context.JournalEntryLines
             .Include(l => l.JournalEntry)
@@ -760,6 +811,24 @@ public class LedgerService : ILedgerService
                 .SumAsync(a => (decimal?)a.AmountApplied) ?? 0;
         }
 
+        // 12c. Sales Discounts contra-revenue (account 4950). Mirrors section 12c in GetAccountLedgerAsync
+        // so the prior-period opening balance matches the actual postings (invoice discounts + memo issues,
+        // less the unapplied remainder of voided memos).
+        if (account.AccountNumber == "4950")
+        {
+            debits += await _context.Invoices
+                .Where(i => i.DiscountAmount > 0
+                         && i.Status != InvoiceStatus.Draft && i.Status != InvoiceStatus.Voided
+                         && i.InvoiceDate < beforeDate)
+                .SumAsync(i => (decimal?)i.DiscountAmount) ?? 0;
+            debits += await _context.CreditMemos
+                .Where(m => m.IssueDate < beforeDate)
+                .SumAsync(m => (decimal?)m.Amount) ?? 0;
+            credits += await _context.CreditMemos
+                .Where(m => m.Status == CreditMemoStatus.Voided && m.UpdatedAt < beforeDate && m.Amount > m.AmountApplied)
+                .SumAsync(m => (decimal?)(m.Amount - m.AmountApplied)) ?? 0;
+        }
+
         // 10. Posted journal entry lines touching this account (prior to period)
         debits += await _context.JournalEntryLines
             .Where(l => l.AccountId == accountId