refactor(infrastructure): Improve service implementations and remove legacy references

**Services Refactored:**
- DevExpressPdfProcessingService: Remove unnecessary try-catch (lines 80-87), add stream position validation
- WindreamDmsService: Mark as [Obsolete] - application now only provides email sending functionality
- MailKitEmailService: Keep MailKit implementation (Limilabs DLL to be added separately)

**Custom Exceptions Added:**
- AuthenticationFailedException: OAuth2/IMAP/SMTP authentication failures
- DmsNotAvailableException: windream COM unavailable
- InvalidPdfException: Invalid PDF stream
- NotFoundException: Entity not found in Repository operations

**Legacy Cleanup:**
- Remove legacy VB.NET projects from solution (EmailProfiler.Common, EmailProfiler.Service)
- Delete legacy/ folder reference
- Clean solution file structure

**Stream Validation:**
- All PDF processing methods now validate stream position (reset to 0 if needed)
- Add CanSeek validation for stream-based operations

**Build Status:** ✅ Successful (0 errors, 15 warnings - all acceptable)

src/DigitalData.EmailProfiler.Infrastructure/Services/DevExpressPdfProcessingService.cs

@@ -0,0 +1,95 @@
+using DevExpress.Pdf;
+using DigitalData.EmailProfiler.Application.Common.Interfaces;
+using DigitalData.EmailProfiler.Domain.Exceptions;
+
+namespace DigitalData.EmailProfiler.Infrastructure.Services;
+
+/// <summary>
+/// PDF processing service using DevExpress.Pdf.
+/// Implements PDF validation and embedded file extraction using streams.
+/// </summary>
+public class DevExpressPdfProcessingService : IPdfProcessingService
+{
+    public Task<bool> ValidatePdfAsync(Stream pdfStream, CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(pdfStream);
+
+        if (!pdfStream.CanRead)
+            throw new ArgumentException("Stream must be readable.", nameof(pdfStream));
+
+        if (!pdfStream.CanSeek)
+            throw new ArgumentException("Stream must be seekable.", nameof(pdfStream));
+
+        if (pdfStream.Position != 0)
+            pdfStream.Position = 0;
+
+        using var processor = new PdfDocumentProcessor();
+        processor.LoadDocument(pdfStream);
+        
+        return Task.FromResult(true);
+    }
+
+    public async Task<IEnumerable<string>> ExtractEmbeddedFilesAsync(
+        Stream pdfStream,
+        string outputDirectory,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(pdfStream);
+        ArgumentException.ThrowIfNullOrWhiteSpace(outputDirectory);
+
+        if (!pdfStream.CanRead)
+            throw new ArgumentException("Stream must be readable.", nameof(pdfStream));
+
+        if (!pdfStream.CanSeek)
+            throw new ArgumentException("Stream must be seekable.", nameof(pdfStream));
+
+        if (pdfStream.Position != 0)
+            pdfStream.Position = 0;
+
+        if (!Directory.Exists(outputDirectory))
+            Directory.CreateDirectory(outputDirectory);
+
+        using var processor = new PdfDocumentProcessor();
+        processor.LoadDocument(pdfStream);
+
+        var extractedFiles = new List<string>();
+        var attachments = processor.Document.FileAttachments;
+
+        if (attachments == null || !attachments.Any())
+            return extractedFiles;
+
+        foreach (var attachment in attachments)
+        {
+            var fileName = attachment.FileName ?? $"attachment_{Guid.NewGuid()}.dat";
+            var outputPath = Path.Combine(outputDirectory, fileName);
+
+            var fileData = attachment.Data;
+            if (fileData == null || fileData.Length == 0)
+                continue;
+
+            await File.WriteAllBytesAsync(outputPath, fileData, cancellationToken);
+            extractedFiles.Add(outputPath);
+        }
+
+        return extractedFiles;
+    }
+
+    public Task<int> GetPageCountAsync(Stream pdfStream, CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(pdfStream);
+
+        if (!pdfStream.CanRead)
+            throw new ArgumentException("Stream must be readable.", nameof(pdfStream));
+
+        if (!pdfStream.CanSeek)
+            throw new ArgumentException("Stream must be seekable.", nameof(pdfStream));
+
+        if (pdfStream.Position != 0)
+            pdfStream.Position = 0;
+
+        using var processor = new PdfDocumentProcessor();
+        processor.LoadDocument(pdfStream);
+
+        return Task.FromResult(processor.Document.Pages.Count);
+    }
+}