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/AttachmentInfo.cs

@@ -0,0 +1,60 @@
+namespace DocumentOperator.Application.Common.DTOs;
+
+/// <summary>
+/// Represents complete attachment information for a PDF document.
+/// Immutable value object containing attachment presence flag, count, and detailed metadata.
+/// </summary>
+public sealed class AttachmentInfo
+{
+    /// <summary>
+    /// Gets a value indicating whether the PDF contains any attachments
+    /// </summary>
+    public bool HasAttachments { get; }
+
+    /// <summary>
+    /// Gets the total number of attachments in the PDF
+    /// </summary>
+    public int AttachmentCount { get; }
+
+    /// <summary>
+    /// Gets the collection of attachment metadata (file details)
+    /// </summary>
+    public IReadOnlyList<AttachmentMetadata> Attachments { get; }
+
+    /// <summary>
+    /// Initializes a new instance of the AttachmentInfo class.
+    /// </summary>
+    /// <param name="hasAttachments">Whether PDF has attachments</param>
+    /// <param name="attachmentCount">Total number of attachments</param>
+    /// <param name="attachments">List of attachment metadata (can be empty)</param>
+    public AttachmentInfo(bool hasAttachments, int attachmentCount, IReadOnlyList<AttachmentMetadata> attachments)
+    {
+        HasAttachments = hasAttachments;
+        AttachmentCount = attachmentCount;
+        Attachments = attachments ?? [];
+
+        // Defensive Programming: Ensure count matches list length
+        if (Attachments.Count != attachmentCount)
+        {
+            throw new ArgumentException(
+                $"Attachment count mismatch: expected {attachmentCount}, got {Attachments.Count}",
+                nameof(attachments));
+        }
+    }
+
+    /// <summary>
+    /// Creates an AttachmentInfo instance for a PDF with no attachments.
+    /// </summary>
+    public static AttachmentInfo Empty =>
+        new(
+            hasAttachments: false,
+            attachmentCount: 0,
+            attachments: []);
+
+    public override string ToString()
+    {
+        return HasAttachments
+            ? $"PDF has {AttachmentCount} attachment(s): {string.Join(", ", Attachments.Select(a => a.FileName))}"
+            : "PDF has no attachments";
+    }
+}

DocumentOperator.Application/Common/DTOs/AttachmentMetadata.cs

@@ -0,0 +1,44 @@
+namespace DocumentOperator.Application.Common.DTOs;
+
+/// <summary>
+/// Represents metadata of a single PDF attachment (embedded file).
+/// Immutable value object containing file information without the actual binary data.
+/// </summary>
+/// <remarks>
+/// Initializes a new instance of the AttachmentMetadata class.
+/// </remarks>
+/// <param name="fileName">Attachment file name</param>
+/// <param name="mimeType">MIME type (e.g., "text/xml")</param>
+/// <param name="sizeBytes">File size in bytes</param>
+public sealed class AttachmentMetadata(string fileName, string mimeType, long sizeBytes)
+{
+    /// <summary>
+    /// Gets the attachment file name (e.g., "invoice.xml", "document.pdf")
+    /// </summary>
+    public string FileName { get; } = fileName ?? string.Empty;
+
+    /// <summary>
+    /// Gets the MIME type of the attachment (e.g., "text/xml", "application/pdf")
+    /// </summary>
+    public string MimeType { get; } = mimeType ?? "application/octet-stream"; // Default MIME type if unknown
+
+    /// <summary>
+    /// Gets the attachment file size in bytes
+    /// </summary>
+    public long SizeBytes { get; } = sizeBytes;
+
+    /// <summary>
+    /// Gets the attachment file size in kilobytes (computed property)
+    /// </summary>
+    public double SizeKB => SizeBytes / 1024.0;
+
+    /// <summary>
+    /// Gets the attachment file size in megabytes (computed property)
+    /// </summary>
+    public double SizeMB => SizeBytes / 1024.0 / 1024.0;
+
+    public override string ToString()
+    {
+        return $"{FileName} ({MimeType}, {SizeKB:F2} KB)";
+    }
+}

DocumentOperator.Application/Common/DTOs/PdfAMetadata.cs

@@ -0,0 +1,34 @@
+namespace DocumentOperator.Application.Common.DTOs;
+
+/// <summary>
+/// PDF/A validation metadata including conformance level and validation errors/warnings
+/// </summary>
+public sealed class PdfAMetadata(
+    bool isValid,
+    string pdfVersion,
+    int pageCount,
+    long fileSizeBytes,
+    bool encrypted,
+    string? pdfaVersion,
+    bool pdfaCompliant,
+    IReadOnlyList<string> errors,
+    IReadOnlyList<string> warnings)
+{
+    public bool IsValid { get; } = isValid;
+    public string PdfVersion { get; } = pdfVersion;
+    public int PageCount { get; } = pageCount;
+    public long FileSizeBytes { get; } = fileSizeBytes;
+    public bool Encrypted { get; } = encrypted;
+    public string? PdfAVersion { get; } = pdfaVersion;
+    public bool PdfACompliant { get; } = pdfaCompliant;
+    public IReadOnlyList<string> Errors { get; } = errors;
+    public IReadOnlyList<string> Warnings { get; } = warnings;
+
+    // Computed property
+    public double FileSizeMB => FileSizeBytes / 1024.0 / 1024.0;
+
+    public override string ToString()
+    {
+        return $"PDF/A: {PdfAVersion ?? "None"}, {PageCount} pages, {FileSizeMB:F2} MB, Compliant: {PdfACompliant}";
+    }
+}

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}";
+    }
+}