Refactor: centralize accounting helpers, status constants, and query deduplication

- AccountingDropdownHelper: wired into BillsController and ExpensesController,
  replacing 35-40 lines of duplicated DB queries per controller
- AppConstants.StatusCodes: added Job.* and Quote.* constants to replace all
  magic status strings across Jobs, Quotes, Appointments, OvenScheduler,
  AiQuickQuote, QuoteApproval, and AccountingDropdownHelper
- AccountingRules: extracted IsNormalDebitBalance into shared Infrastructure
  helper; removed duplicate private method from AccountBalanceService and
  LedgerService (~50 lines deleted)
- AccountDataExportController: extracted 9 Fetch*Async methods (superset of
  includes) so Add*Sheet and Build*Csv no longer duplicate DB queries; each
  entity is queried once regardless of whether XLSX or CSV format is requested
- BillsController.Create and ExpensesController.Create wrapped in
  ExecuteInTransactionAsync; blob uploads moved after commit to keep
  financial data atomic and prevent orphaned blobs from rolling back
- Number generators (Appointments, CreditMemo, OvenBatch) fixed from full-table
  GetAllAsync to prefix-filtered FindAsync

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

src/PowderCoating.Infrastructure/Services/AccountingRules.cs

@@ -0,0 +1,41 @@
+using PowderCoating.Core.Enums;
+
+namespace PowderCoating.Infrastructure.Services;
+
+/// <summary>
+/// Single source of truth for double-entry sign conventions shared by
+/// <see cref="AccountBalanceService"/> and <see cref="LedgerService"/>.
+/// Centralised here so that adding a new AccountSubType only requires
+/// one edit rather than two independently maintained switch expressions.
+/// </summary>
+internal static class AccountingRules
+{
+    /// <summary>
+    /// Returns <c>true</c> for sub-types whose normal balance is a debit
+    /// (Assets, COGS, Expenses). Sub-type is used rather than AccountType
+    /// because it is constrained to a known enum set and cannot be
+    /// misconfigured by a user. Expense enum values are ≥ 50 by convention,
+    /// allowing a catch-all range match for any future expense sub-types.
+    /// </summary>
+    internal static bool IsNormalDebitBalance(AccountSubType subType) => subType switch
+    {
+        // Asset subtypes → normal debit balance
+        AccountSubType.Cash
+            or AccountSubType.Checking
+            or AccountSubType.Savings
+            or AccountSubType.AccountsReceivable
+            or AccountSubType.Inventory
+            or AccountSubType.FixedAsset
+            or AccountSubType.OtherCurrentAsset
+            or AccountSubType.OtherAsset => true,
+
+        // COGS → normal debit balance
+        AccountSubType.CostOfGoodsSold => true,
+
+        // Expense subtypes (enum values ≥ 50) → normal debit balance
+        var st when (int)st >= 50 => true,
+
+        // Liability subtypes (AP, CreditCard, etc.), Equity, Revenue → normal credit balance
+        _ => false
+    };
+}