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/Controllers/CustomersController.cs

@@ -943,13 +943,18 @@ public class CustomersController : Controller
     /// </summary>
     private async Task<string> GenerateCreditMemoNumberAsync()
     {
-        var allMemos = await _unitOfWork.CreditMemos.GetAllAsync(true);
         var prefix = $"CM-{DateTime.Now:yyMM}-";
-        var maxNum = allMemos
-            .Where(m => m.MemoNumber.StartsWith(prefix))
-            .Select(m => { int.TryParse(m.MemoNumber.Replace(prefix, ""), out int n); return n; })
-            .DefaultIfEmpty(0).Max();
-        return $"{prefix}{(maxNum + 1):D4}";
+        var last = (await _unitOfWork.CreditMemos.FindAsync(
+                m => m.MemoNumber.StartsWith(prefix), ignoreQueryFilters: true))
+            .OrderByDescending(m => m.MemoNumber)
+            .Select(m => m.MemoNumber)
+            .FirstOrDefault();
+
+        int next = 1;
+        if (last != null && int.TryParse(last[prefix.Length..], out int num))
+            next = num + 1;
+
+        return $"{prefix}{next:D4}";
     }
 
     /// <summary>