Close all GL entry gaps across the accounting surface

- Stripe payments/refunds/chargebacks now post DR/CR entries (PaymentController)
- Vendor credit void now reverses the posted GL lines (VendorCreditsController)
- Gift certificate issue/redeem/void post GL to account 2500 GC Liability;
  FinancialReportService Trial Balance + Balance Sheet include GC liability and
  breakage income; P&L shows deferred revenue deduction and breakage income line
- Customer deposits now post DR Checking / CR 2300 on record, reverse on delete;
  invoice auto-apply uses DR 2300 / CR AR (not a second bank debit); draft
  invoice delete reverses deposit-apply GL before the AR reversal
- Deposit.DepositAccountId column added; account 2300 seeded via migration
- InvoicesController.ApplyCredit now posts DR Sales Discounts / CR AR,
  consistent with CreditMemosController.Apply
- IssueRefund (cash/card) posts DR AR / CR Bank and sets Refund.DepositAccountId;
  refund modal gains a bank account selector hidden for store-credit path
- CancelRefund (cash/card) reverses the IssueRefund GL entries
- LedgerService GetAccountLedgerAsync + ComputePriorBalanceAsync now include
  Refunds, CreditMemoApplications, VendorCreditApplications, GC Liability (2500),
  and Customer Deposits (2300) so account ledger view and RecalculateAllAsync
  produce correct balances
- Three EF migrations applied: SeedSalesDiscountsAccount, AccountingGapsPhase2,
  AccountingDepositsGL
- Unit tests updated for new IAccountBalanceService constructor params (200/200)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Infrastructure/Services/SeedDataService.Accounts.cs

@@ -70,6 +70,10 @@ public partial class SeedDataService
             new Account { AccountNumber = "4100", Name = "Sandblasting Revenue",      AccountType = AccountType.Revenue,    AccountSubType = AccountSubType.ServiceRevenue,      IsSystem = false, IsActive = true, Description = "Revenue from sandblasting services",         CompanyId = company.Id, CreatedAt = now },
             new Account { AccountNumber = "4200", Name = "Other Service Revenue",     AccountType = AccountType.Revenue,    AccountSubType = AccountSubType.ServiceRevenue,      IsSystem = false, IsActive = true, Description = "Revenue from other shop services",           CompanyId = company.Id, CreatedAt = now },
             new Account { AccountNumber = "4900", Name = "Other Income",              AccountType = AccountType.Revenue,    AccountSubType = AccountSubType.OtherIncome,         IsSystem = false, IsActive = true, Description = "Miscellaneous income",                       CompanyId = company.Id, CreatedAt = now },
+            // Contra-revenue: debited when invoice discounts are applied so the GL balances.
+            // A credit-normal account with a debit balance appears in the Trial Balance debit column,
+            // reducing net revenue to match the discounted AR amount that was posted.
+            new Account { AccountNumber = "4950", Name = "Sales Discounts",           AccountType = AccountType.Revenue,    AccountSubType = AccountSubType.OtherIncome,         IsSystem = true,  IsActive = true, Description = "Contra-revenue for invoice discounts granted to customers", CompanyId = company.Id, CreatedAt = now },
 
             // ── COST OF GOODS SOLD ────────────────────────────────────────────
             new Account { AccountNumber = "5000", Name = "Cost of Goods Sold",        AccountType = AccountType.CostOfGoods, AccountSubType = AccountSubType.CostOfGoodsSold,   IsSystem = false, IsActive = true, Description = "Direct cost of services delivered",          CompanyId = company.Id, CreatedAt = now },
@@ -96,4 +100,44 @@ public partial class SeedDataService
 
         return accounts.Count;
     }
+
+    /// <summary>
+    /// Ensures system accounts introduced after the initial chart-of-accounts seed exist for the
+    /// given company. Idempotent: each account is only inserted when absent, so this is safe to
+    /// call repeatedly from the "Seed Lookup Tables" flow.
+    /// Call this after <see cref="SeedDefaultChartOfAccountsAsync"/> so that newly onboarded
+    /// companies get all accounts in one pass while existing companies receive only the missing ones.
+    /// </summary>
+    /// <returns>Number of accounts inserted (0 if all are already present).</returns>
+    private async Task<int> EnsureSystemAccountsAsync(Company company)
+    {
+        var now = DateTime.UtcNow;
+        int added = 0;
+
+        // 4950 Sales Discounts — contra-revenue account introduced to balance the GL when
+        // invoice discounts are applied (DR Sales Discounts / CR Revenue gap fixed).
+        var has4950 = await _context.Set<Account>()
+            .IgnoreQueryFilters()
+            .AnyAsync(a => a.CompanyId == company.Id && a.AccountNumber == "4950" && !a.IsDeleted);
+
+        if (!has4950)
+        {
+            _context.Set<Account>().Add(new Account
+            {
+                AccountNumber  = "4950",
+                Name           = "Sales Discounts",
+                AccountType    = AccountType.Revenue,
+                AccountSubType = AccountSubType.OtherIncome,
+                IsSystem       = true,
+                IsActive       = true,
+                Description    = "Contra-revenue for invoice discounts granted to customers",
+                CompanyId      = company.Id,
+                CreatedAt      = now
+            });
+            await _context.SaveChangesAsync();
+            added++;
+        }
+
+        return added;
+    }
 }