Complete Phase 2: Domain Layer implementation

Updated ROADMAP.md to mark Phase 2 as completed and added detailed descriptions of completed tasks. Introduced three new value objects (`Base64String`, `TenantId`, and `PdfMetadata`) in the `DocumentOperator.Domain.Models.ValueObjects` namespace. These classes ensure type safety, immutability, and encapsulated validation.

- `Base64String`: Handles Base64 string creation, validation, and conversion.
- `TenantId`: Represents a tenant identifier with validation and normalization.
- `PdfMetadata`: Represents PDF metadata with computed properties.

Updated `DocumentOperator.Domain.csproj` to reflect the addition of these value objects. The project is now ready to begin Phase 3 (Infrastructure Layer).

DocumentOperator.Domain/Models/ValueObjects/PdfMetadata.cs

@@ -0,0 +1,32 @@
+namespace DocumentOperator.Domain.Models.ValueObjects;
+
+public sealed class PdfMetadata
+{
+    public int PageCount { get; }
+    public long FileSizeBytes { get; }
+    public string PdfVersion { get; }
+    public bool HasAttachments { get; }
+    public int AttachmentCount { get; }
+
+    // Computed Property (berechnet aus FileSizeBytes)
+    public double FileSizeMB => FileSizeBytes / 1024.0 / 1024.0;
+
+    public PdfMetadata(
+        int pageCount,
+        long fileSizeBytes,
+        string pdfVersion,
+        bool hasAttachments,
+        int attachmentCount)
+    {
+        PageCount = pageCount;
+        FileSizeBytes = fileSizeBytes;
+        PdfVersion = pdfVersion;
+        HasAttachments = hasAttachments;
+        AttachmentCount = attachmentCount;
+    }
+
+    public override string ToString()
+    {
+        return $"PDF: {PageCount} pages, {FileSizeMB:F2} MB, Version {PdfVersion}, Attachments: {AttachmentCount}";
+    }
+}