feat(04-01): add Phase 4 models, interfaces, BulkOperationRunner, and test scaffolds

- Install CsvHelper 33.1.0 and Microsoft.Graph 5.74.0 (main + test projects)
- Add 14 core model/enum files (BulkOperationResult, BulkMemberRow, BulkSiteRow, TransferJob, FolderStructureRow, SiteTemplate, SiteTemplateOptions, TemplateLibraryInfo, TemplateFolderInfo, TemplatePermissionGroup, ConflictPolicy, TransferMode, CsvValidationRow)
- Add 6 service interfaces (IFileTransferService, IBulkMemberService, IBulkSiteService, ITemplateService, IFolderStructureService, ICsvValidationService)
- Add BulkOperationRunner with continue-on-error and cancellation support
- Add BulkResultCsvExportService stub (compile-ready)
- Add test scaffolds: BulkOperationRunnerTests (5 passing), BulkResultCsvExportServiceTests (2 passing), CsvValidationServiceTests (6 skipped), TemplateRepositoryTests (4 skipped)

SharepointToolbox/Core/Models/BulkMemberRow.cs

@@ -0,0 +1,18 @@
+using CsvHelper.Configuration.Attributes;
+
+namespace SharepointToolbox.Core.Models;
+
+public class BulkMemberRow
+{
+    [Name("GroupName")]
+    public string GroupName { get; set; } = string.Empty;
+
+    [Name("GroupUrl")]
+    public string GroupUrl { get; set; } = string.Empty;
+
+    [Name("Email")]
+    public string Email { get; set; } = string.Empty;
+
+    [Name("Role")]
+    public string Role { get; set; } = string.Empty; // "Member" or "Owner"
+}