feat(1099): round-trip Form1099BoxId through expense category DTOs/service

Mirror Form990LineId: add Form1099BoxId + Form1099BoxCode to all four
category DTOs (response + request, group + sub); load a boxCodes lookup
dictionary in GetAllAsync and project it; set/copy the field in
CreateGroupAsync, UpdateGroupAsync, CreateSubCategoryAsync, and
UpdateSubCategoryAsync. All 4 category-service unit tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API/ROLAC.API/DTOs/Expense/ExpenseCategoryDtos.cs

@@ -11,6 +11,8 @@ public class ExpenseSubCategoryDto
     public bool    IsActive  { get; set; }
     public int?    Form990LineId   { get; set; }
     public string? Form990LineCode { get; set; }
+    public int?    Form1099BoxId   { get; set; }
+    public string? Form1099BoxCode { get; set; }
 }
 
 public class ExpenseCategoryGroupDto
@@ -22,6 +24,8 @@ public class ExpenseCategoryGroupDto
     public bool    IsActive  { get; set; }
     public int?    Form990LineId   { get; set; }
     public string? Form990LineCode { get; set; }
+    public int?    Form1099BoxId   { get; set; }
+    public string? Form1099BoxCode { get; set; }
     public List<ExpenseSubCategoryDto> SubCategories { get; set; } = [];
 }
 
@@ -31,6 +35,7 @@ public class CreateExpenseGroupRequest
     [MaxLength(200)] public string? Name_zh { get; set; }
     public int SortOrder { get; set; }
     public int? Form990LineId { get; set; }
+    public int? Form1099BoxId { get; set; }
 }
 public class UpdateExpenseGroupRequest : CreateExpenseGroupRequest
 {
@@ -44,6 +49,7 @@ public class CreateExpenseSubCategoryRequest
     [MaxLength(200)] public string? Name_zh { get; set; }
     public int SortOrder { get; set; }
     public int? Form990LineId { get; set; }
+    public int? Form1099BoxId { get; set; }
 }
 public class UpdateExpenseSubCategoryRequest : CreateExpenseSubCategoryRequest
 {