feat: enforce tenant isolation and shared question bank

Tiku.DbMigrator/Program.cs

@@ -4,6 +4,7 @@ using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
 using Tiku.Infrastructure;
 using Tiku.Infrastructure.Persistence;
+using Tiku.Application;
 
 var builder = Host.CreateApplicationBuilder(args);
 var connectionString =
@@ -12,6 +13,7 @@ var connectionString =
     throw new InvalidOperationException(
         "Database connection is required. Configure ConnectionStrings:Database or DATABASE_URL.");
 
+builder.Services.AddApplication();
 builder.Services.AddInfrastructure(connectionString);
 
 using var host = builder.Build();