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/DataProtectionEncryptionService.cs

@@ -0,0 +1,37 @@
+using DigitalData.EmailProfiler.Application.Common.Interfaces;
+using Microsoft.AspNetCore.DataProtection;
+
+namespace DigitalData.EmailProfiler.Infrastructure.Services;
+
+/// <summary>
+/// Encryption service using ASP.NET Core Data Protection API.
+/// Passwords are encrypted at rest in the database.
+/// </summary>
+public class DataProtectionEncryptionService(IDataProtectionProvider Provider) : IEncryptionService
+{
+    private readonly IDataProtector Protector = Provider.CreateProtector("EmailProfiler.Passwords");
+
+    public string Encrypt(string plainText)
+    {
+        if (string.IsNullOrEmpty(plainText))
+            return string.Empty;
+
+        return Protector.Protect(plainText);
+    }
+
+    public string Decrypt(string cipherText)
+    {
+        if (string.IsNullOrEmpty(cipherText))
+            return string.Empty;
+
+        try
+        {
+            return Protector.Unprotect(cipherText);
+        }
+        catch (Exception)
+        {
+            // If decryption fails, return empty (corrupt data or wrong key)
+            return string.Empty;
+        }
+    }
+}