feat(03-01): create 7 core models and 3 service interfaces for Phase 3

- StorageNode, StorageScanOptions models
- SearchResult, SearchOptions models
- DuplicateItem, DuplicateGroup, DuplicateScanOptions models
- IStorageService, ISearchService, IDuplicatesService interfaces

SharepointToolbox/Core/Models/StorageScanOptions.cs

@@ -0,0 +1,7 @@
+namespace SharepointToolbox.Core.Models;
+
+public record StorageScanOptions(
+    bool PerLibrary = true,
+    bool IncludeSubsites = false,
+    int FolderDepth = 0   // 0 = library root only; >0 = recurse N levels
+);