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/Queue/InMemoryEmailQueue.cs

@@ -0,0 +1,48 @@
+using System.Threading.Channels;
+using DigitalData.EmailProfiler.Application.Common.Interfaces;
+using DigitalData.EmailProfiler.Domain.Entities;
+
+namespace DigitalData.EmailProfiler.Infrastructure.Queue;
+
+/// <summary>
+/// In-memory email queue implementation using System.Threading.Channels.
+/// Thread-safe, high-performance queue for outgoing emails.
+/// TODO: Replace with RabbitMQ for production (see AGENTS.md Section 7).
+/// </summary>
+public class InMemoryEmailQueue : IEmailQueue
+{
+    private readonly Channel<EmailOutbox> _channel;
+
+    public InMemoryEmailQueue()
+    {
+        var options = new BoundedChannelOptions(1000)
+        {
+            FullMode = BoundedChannelFullMode.Wait
+        };
+
+        _channel = Channel.CreateBounded<EmailOutbox>(options);
+    }
+
+    public async Task EnqueueAsync(EmailOutbox email, CancellationToken cancellationToken = default)
+    {
+        await _channel.Writer.WriteAsync(email, cancellationToken);
+    }
+
+    public async Task<EmailOutbox?> DequeueAsync(CancellationToken cancellationToken = default)
+    {
+        if (await _channel.Reader.WaitToReadAsync(cancellationToken))
+        {
+            if (_channel.Reader.TryRead(out var email))
+            {
+                return email;
+            }
+        }
+
+        return null;
+    }
+
+    public Task<int> GetQueueDepthAsync(CancellationToken cancellationToken = default)
+    {
+        return Task.FromResult(_channel.Reader.Count);
+    }
+}