Add EF Core In-Memory DB and repository pattern support

Added the `Microsoft.EntityFrameworkCore.InMemory` package to enable an in-memory database for testing and lightweight storage. Introduced `MessagingServiceDbContext` with `DbSet` properties for `EmailAccount`, `ReceivedEmail`, and `EmailAttachment`. Configured entity relationships in `OnModelCreating`.

Registered `MessagingServiceDbContext` and a generic repository (`IRepository<>`) in the dependency injection container. Updated namespaces and imports to support the new DbContext and repository.

Improved project structure to enhance modularity and testability by leveraging EF Core's in-memory database.

src/infrastructure/DigitalData.MessagingService.Infrastructure/DependencyInjection.cs

@@ -1,10 +1,14 @@
 using DigitalData.MessagingService.Application.Common.Interfaces;
+using DigitalData.MessagingService.Application.Common.Interfaces.Repositories;
+using DigitalData.MessagingService.Infrastructure.Persistence;
 using DigitalData.MessagingService.Infrastructure.Queue;
+using DigitalData.MessagingService.Infrastructure.Repositories;
 using DigitalData.MessagingService.Infrastructure.Services;
 using DigitalData.MessagingService.Infrastructure.Services.Background;
 using DigitalData.MessagingService.Publisher;
 using DigitalData.MessagingService.RabbitMQ;
 using Microsoft.AspNetCore.DataProtection;
+using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 
@@ -54,6 +58,13 @@ public static class DependencyInjection
 
         services.AddMemoryCache();
 
+        // --- Database (InMemory) ---
+        services.AddDbContext<MessagingServiceDbContext>(options =>
+            options.UseInMemoryDatabase("MessagingServiceDb"));
+
+        services.AddScoped(typeof(IRepository<>), typeof(Repository<>));
+
         return services;
     }
 }
+