Add new Backend structure with proper .NET 8 projects

src/Backend/DiunaBI.Database/Context/DesignTimeDbContextFactory.cs

@@ -0,0 +1,31 @@
+using System;
+using System.IO;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Design;
+using Microsoft.Extensions.Configuration;
+
+namespace DiunaBI.Database.Context;
+
+public class DesignTimeDbContextFactory : IDesignTimeDbContextFactory<AppDbContext>
+{
+    public AppDbContext CreateDbContext(string[] args)
+    {
+        var configuration = new ConfigurationBuilder()
+            .SetBasePath(Path.Combine(Directory.GetCurrentDirectory(), "../DiunaBI.WebAPI"))  // ✅ Poprawna ścieżka
+            .AddJsonFile("appsettings.json", optional: false)
+            .AddJsonFile("appsettings.Development.json", optional: true)
+            .Build();
+
+        var optionsBuilder = new DbContextOptionsBuilder<AppDbContext>();
+
+        var connectionString = configuration.GetConnectionString("DefaultConnection");
+        if (string.IsNullOrEmpty(connectionString))
+        {
+            throw new InvalidOperationException("Connection string 'DefaultConnection' not found in appsettings.json");
+        }
+
+        optionsBuilder.UseSqlServer(connectionString);
+
+        return new AppDbContext(optionsBuilder.Options);
+    }
+}