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/Persistence/EmailProfilerDbContext.cs

@@ -0,0 +1,21 @@
+using DigitalData.EmailProfiler.Domain.Entities;
+using Microsoft.EntityFrameworkCore;
+
+namespace DigitalData.EmailProfiler.Infrastructure.Persistence;
+
+/// <summary>
+/// Entity Framework Core DbContext for EmailProfiler.
+/// IMPORTANT: This context maps to a LEGACY database - NO schema modifications allowed!
+/// </summary>
+public class EmailProfilerDbContext(DbContextOptions<EmailProfilerDbContext> options) : DbContext(options)
+{
+    // DbSets for all entities
+    public DbSet<EmailAccount> EmailAccounts { get; set; }
+    public DbSet<EmailProfile> EmailProfiles { get; set; }
+    public DbSet<EmailHistory> EmailHistories { get; set; }
+    public DbSet<EmailAttachment> EmailAttachments { get; set; }
+    public DbSet<EmailProcess> EmailProcesses { get; set; }
+    public DbSet<ProcessStep> ProcessSteps { get; set; }
+    public DbSet<IndexingStep> IndexingSteps { get; set; }
+    public DbSet<EmailOutbox> EmailOutbox { get; set; }
+}