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/SeedDataService.Accounts.cs

@@ -61,6 +61,7 @@ public partial class SeedDataService
             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 },
+            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 = "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 ────────────────────────────────────────────────────────
@@ -166,6 +167,30 @@ public partial class SeedDataService
             added++;
         }
 
+        // 2350 Customer Credits — liability for store credit owed to customers. Credited when a credit
+        // memo (incl. store-credit refunds) is issued; debited when applied to an invoice.
+        var has2350 = await _context.Set<Account>()
+            .IgnoreQueryFilters()
+            .AnyAsync(a => a.CompanyId == company.Id && a.AccountNumber == "2350" && !a.IsDeleted);
+
+        if (!has2350)
+        {
+            _context.Set<Account>().Add(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
+            });
+            await _context.SaveChangesAsync();
+            added++;
+        }
+
         return added;
     }
 }