refactor: rename namespace to FiscalFlow and upgrade to .NET 10

- Rename InvoiceMaster.* to FiscalFlow.* namespace
- Upgrade from .NET 8 to .NET 10
- Update all NuGet packages to latest versions
- Update C# language version to 14.0

backend/tests/FiscalFlow.IntegrationTests/Factories/CustomWebApplicationFactory.cs

@@ -0,0 +1,35 @@
+using FiscalFlow.API;
+using FiscalFlow.Infrastructure.Data;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Mvc.Testing;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace FiscalFlow.IntegrationTests.Factories;
+
+public class CustomWebApplicationFactory : WebApplicationFactory<Program>
+{
+    protected override void ConfigureWebHost(IWebHostBuilder builder)
+    {
+        builder.ConfigureServices(services =>
+        {
+            var descriptor = services.SingleOrDefault(
+                d => d.ServiceType == typeof(DbContextOptions<ApplicationDbContext>));
+
+            if (descriptor != null)
+            {
+                services.Remove(descriptor);
+            }
+
+            services.AddDbContext<ApplicationDbContext>(options =>
+            {
+                options.UseInMemoryDatabase("TestDb");
+            });
+
+            var sp = services.BuildServiceProvider();
+            using var scope = sp.CreateScope();
+            var db = scope.ServiceProvider.GetRequiredService<ApplicationDbContext>();
+            db.Database.EnsureCreated();
+        });
+    }
+}