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/SeedDataService.Accounts.cs

@@ -60,8 +60,12 @@ public partial class SeedDataService
             new Account { AccountNumber = "2000", Name = "Accounts Payable",          AccountType = AccountType.Liability,  AccountSubType = AccountSubType.AccountsPayable,     IsSystem = true,  IsActive = true, Description = "Amounts owed to suppliers and vendors",      CompanyId = company.Id, CreatedAt = now },
             new Account { AccountNumber = "2100", Name = "Credit Card Payable",       AccountType = AccountType.Liability,  AccountSubType = AccountSubType.CreditCard,          IsSystem = false, IsActive = true, Description = "Business credit card balance",               CompanyId = company.Id, CreatedAt = now },
             new Account { AccountNumber = "2200", Name = "Sales Tax Payable",         AccountType = AccountType.Liability,  AccountSubType = AccountSubType.OtherCurrentLiability, IsSystem = false, IsActive = true, Description = "Sales tax collected and owed to government", CompanyId = company.Id, CreatedAt = now },
-            new Account { AccountNumber = "2300", Name = "Payroll Liabilities",       AccountType = AccountType.Liability,  AccountSubType = AccountSubType.OtherCurrentLiability, IsSystem = false, IsActive = true, Description = "Payroll taxes and withholdings owed",       CompanyId = company.Id, CreatedAt = now },
+            // 2300 is the Customer Deposits liability — credited when a deposit is taken, debited when it is
+            // applied to an invoice (see DepositsController / InvoicesController, which resolve it by number).
+            // IsSystem because the GL posting code depends on it existing. Payroll lives at 2400 below.
+            new Account { AccountNumber = "2300", Name = "Customer Deposits",          AccountType = AccountType.Liability,  AccountSubType = AccountSubType.OtherCurrentLiability, IsSystem = true,  IsActive = true, Description = "Deposits received from customers before an invoice is created; cleared when applied to an invoice", CompanyId = company.Id, CreatedAt = now },
             new Account { AccountNumber = "2350", Name = "Customer Credits",           AccountType = AccountType.Liability,  AccountSubType = AccountSubType.OtherCurrentLiability, IsSystem = true,  IsActive = true, Description = "Store credit owed to customers (credit memos not yet applied)", CompanyId = company.Id, CreatedAt = now },
+            new Account { AccountNumber = "2400", Name = "Payroll Liabilities",        AccountType = AccountType.Liability,  AccountSubType = AccountSubType.OtherCurrentLiability, IsSystem = false, IsActive = true, Description = "Payroll taxes and withholdings owed",       CompanyId = company.Id, CreatedAt = now },
             new Account { AccountNumber = "2900", Name = "Business Loan",             AccountType = AccountType.Liability,  AccountSubType = AccountSubType.LongTermLiability,   IsSystem = false, IsActive = true, Description = "Long-term equipment or business loan",       CompanyId = company.Id, CreatedAt = now },
 
             // ── EQUITY ────────────────────────────────────────────────────────
@@ -191,6 +195,46 @@ public partial class SeedDataService
             added++;
         }
 
+        // 2300 used to be seeded as "Payroll Liabilities" but the deposit GL posting code has always
+        // resolved 2300 by number and used it as the Customer Deposits liability — so the account was
+        // mislabeled on the balance sheet. Rename it to "Customer Deposits" and mark it system. Only
+        // touch accounts still carrying the old default name so a user's own rename is preserved.
+        var legacyDepositsAcct = await _context.Set<Account>()
+            .IgnoreQueryFilters()
+            .FirstOrDefaultAsync(a => a.CompanyId == company.Id && a.AccountNumber == "2300"
+                                   && !a.IsDeleted && a.Name == "Payroll Liabilities");
+        if (legacyDepositsAcct != null)
+        {
+            legacyDepositsAcct.Name        = "Customer Deposits";
+            legacyDepositsAcct.Description = "Deposits received from customers before an invoice is created; cleared when applied to an invoice";
+            legacyDepositsAcct.IsSystem    = true;
+            legacyDepositsAcct.UpdatedAt   = now;
+            await _context.SaveChangesAsync();
+        }
+
+        // 2400 Payroll Liabilities — the payroll account displaced from 2300 (now Customer Deposits).
+        var has2400 = await _context.Set<Account>()
+            .IgnoreQueryFilters()
+            .AnyAsync(a => a.CompanyId == company.Id && a.AccountNumber == "2400" && !a.IsDeleted);
+
+        if (!has2400)
+        {
+            _context.Set<Account>().Add(new Account
+            {
+                AccountNumber  = "2400",
+                Name           = "Payroll Liabilities",
+                AccountType    = AccountType.Liability,
+                AccountSubType = AccountSubType.OtherCurrentLiability,
+                IsSystem       = false,
+                IsActive       = true,
+                Description    = "Payroll taxes and withholdings owed",
+                CompanyId      = company.Id,
+                CreatedAt      = now
+            });
+            await _context.SaveChangesAsync();
+            added++;
+        }
+
         return added;
     }
 }