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/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -7241,7 +7241,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 1,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 6, 17, 19, 26, 43, 161, DateTimeKind.Utc).AddTicks(2051),
+                            CreatedAt = new DateTime(2026, 6, 19, 23, 31, 4, 905, DateTimeKind.Utc).AddTicks(7611),
                             Description = "Standard pricing for regular customers",
                             DiscountPercent = 0m,
                             IsActive = true,
@@ -7252,7 +7252,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 2,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 6, 17, 19, 26, 43, 161, DateTimeKind.Utc).AddTicks(2056),
+                            CreatedAt = new DateTime(2026, 6, 19, 23, 31, 4, 905, DateTimeKind.Utc).AddTicks(7618),
                             Description = "5% discount for preferred customers",
                             DiscountPercent = 5m,
                             IsActive = true,
@@ -7263,7 +7263,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 3,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 6, 17, 19, 26, 43, 161, DateTimeKind.Utc).AddTicks(2057),
+                            CreatedAt = new DateTime(2026, 6, 19, 23, 31, 4, 905, DateTimeKind.Utc).AddTicks(7619),
                             Description = "10% discount for premium customers",
                             DiscountPercent = 10m,
                             IsActive = true,