feat(02-02): define PermissionEntry, ScanOptions, and IPermissionsService

- PermissionEntry record with 9 fields matching PS Generate-PnPSitePermissionRpt
- ScanOptions record with defaults: IncludeInherited=false, ScanFolders=true, FolderDepth=1, IncludeSubsites=false
- IPermissionsService interface with ScanSiteAsync method enabling ViewModel mocking

SharepointToolbox/Core/Models/ScanOptions.cs

@@ -0,0 +1,12 @@
+namespace SharepointToolbox.Core.Models;
+
+/// <summary>
+/// Immutable scan configuration value object.
+/// Controls which SharePoint objects are included in the permission scan.
+/// </summary>
+public record ScanOptions(
+    bool IncludeInherited = false,  // When false: only objects with unique permissions are returned
+    bool ScanFolders      = true,   // Include folder-level permission entries
+    int  FolderDepth      = 1,      // Max folder depth to scan (999 = unlimited)
+    bool IncludeSubsites  = false   // Whether to recursively scan subsites
+);