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/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}";
+    }
+}