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.Domain/Exceptions/AuthenticationFailedException.cs

@@ -0,0 +1,17 @@
+namespace DigitalData.EmailProfiler.Domain.Exceptions;
+
+/// <summary>
+/// Exception thrown when OAuth2 authentication fails.
+/// </summary>
+public class AuthenticationFailedException : Exception
+{
+    public AuthenticationFailedException(string message)
+        : base(message)
+    {
+    }
+
+    public AuthenticationFailedException(string message, Exception innerException)
+        : base(message, innerException)
+    {
+    }
+}

src/DigitalData.EmailProfiler.Domain/Exceptions/DmsNotAvailableException.cs

@@ -0,0 +1,22 @@
+namespace DigitalData.EmailProfiler.Domain.Exceptions;
+
+/// <summary>
+/// Exception thrown when DMS (windream) is not available or not configured properly.
+/// </summary>
+public class DmsNotAvailableException : Exception
+{
+    public DmsNotAvailableException()
+        : base("DMS service is not available. windream COM components may not be registered.")
+    {
+    }
+
+    public DmsNotAvailableException(string message)
+        : base(message)
+    {
+    }
+
+    public DmsNotAvailableException(string message, Exception innerException)
+        : base(message, innerException)
+    {
+    }
+}

src/DigitalData.EmailProfiler.Domain/Exceptions/InvalidPdfException.cs

@@ -0,0 +1,17 @@
+namespace DigitalData.EmailProfiler.Domain.Exceptions;
+
+/// <summary>
+/// Exception thrown when a PDF file is invalid or corrupted.
+/// </summary>
+public class InvalidPdfException : Exception
+{
+    public InvalidPdfException(string message)
+        : base(message)
+    {
+    }
+
+    public InvalidPdfException(string message, Exception innerException)
+        : base(message, innerException)
+    {
+    }
+}

src/DigitalData.EmailProfiler.Domain/Exceptions/NotFoundException.cs

@@ -0,0 +1,22 @@
+namespace DigitalData.EmailProfiler.Domain.Exceptions;
+
+/// <summary>
+/// Exception thrown when a requested entity is not found.
+/// </summary>
+public class NotFoundException : Exception
+{
+    public NotFoundException(string entityName, object key)
+        : base($"{entityName} with key '{key}' was not found.")
+    {
+    }
+
+    public NotFoundException(string message)
+        : base(message)
+    {
+    }
+
+    public NotFoundException(string message, Exception innerException)
+        : base(message, innerException)
+    {
+    }
+}