refactor: Move DTOs from Domain to Application layer

- Move PdfMetadata, PdfAMetadata from Domain.Models.ValueObjects to Application.Common.DTOs
- Move AttachmentInfo, AttachmentMetadata from Domain.Models.ValueObjects to Application.Common.DTOs
- Reason: DTOs belong in Application layer, Domain should have zero external dependencies (Clean Architecture)

DocumentOperator.Application/Common/DTOs/PdfMetadata.cs

@@ -0,0 +1,23 @@
+namespace DocumentOperator.Application.Common.DTOs;
+
+public sealed class PdfMetadata(
+    int pageCount,
+    long fileSizeBytes,
+    string pdfVersion,
+    bool hasAttachments,
+    int attachmentCount)
+{
+    public int PageCount { get; } = pageCount;
+    public long FileSizeBytes { get; } = fileSizeBytes;
+    public string PdfVersion { get; } = pdfVersion;
+    public bool HasAttachments { get; } = hasAttachments;
+    public int AttachmentCount { get; } = attachmentCount;
+
+    // Computed Property (berechnet aus FileSizeBytes)
+    public double FileSizeMB => FileSizeBytes / 1024.0 / 1024.0;
+
+    public override string ToString()
+    {
+        return $"PDF: {PageCount} pages, {FileSizeMB:F2} MB, Version {PdfVersion}, Attachments: {AttachmentCount}";
+    }
+}