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.Web/Helpers/AccountingDropdownHelper.cs

@@ -1,6 +1,7 @@
-using Microsoft.AspNetCore.Mvc.Rendering;
+using Microsoft.AspNetCore.Mvc.Rendering;
 using PowderCoating.Core.Enums;
 using PowderCoating.Core.Interfaces;
+using PowderCoating.Shared.Constants;
 
 namespace PowderCoating.Web.Helpers;
 
@@ -21,9 +22,9 @@ internal static class AccountingDropdownHelper
         var vendors = await unitOfWork.Vendors.FindAsync(v => v.IsActive);
         var allAccounts = await unitOfWork.Accounts.FindAsync(a => a.IsActive);
         var jobs = await unitOfWork.Jobs.FindAsync(j =>
-            j.JobStatus.StatusCode != "COMPLETED" &&
-            j.JobStatus.StatusCode != "CANCELLED" &&
-            j.JobStatus.StatusCode != "DELIVERED");
+            j.JobStatus.StatusCode != AppConstants.StatusCodes.Job.Completed &&
+            j.JobStatus.StatusCode != AppConstants.StatusCodes.Job.Cancelled &&
+            j.JobStatus.StatusCode != AppConstants.StatusCodes.Job.Delivered);
 
         var accountLabel = (Core.Entities.Account a) => $"{a.AccountNumber} – {a.Name}";