refactor(application): introduce MessagingServiceApplicationConfiguration builder

Replace the implicit IConfiguration-based DI registration with an explicit
builder that validates required steps at startup.

- Add MessagingServiceApplicationConfiguration with mandatory steps:
    SetLuckyPennySoftLicenseKey(), ConfigureEmailAccounts() / PublishOnly()
- EnsureConfigured() throws at registration time if either step is missing,
  surfacing misconfiguration before the app starts
- Keep the IConfiguration convenience overload as a thin wrapper over the
  builder for backward-compatible call sites
- Move DependencyInjection.cs into a DependencyInjection/ subfolder alongside
  the new configuration class

src/core/DigitalData.MessagingService.Application/DependencyInjection/DependencyInjection.cs

@@ -0,0 +1,51 @@
+#if NET
+using FluentValidation;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using System.Reflection;
+
+namespace DigitalData.MessagingService.Application.DependencyInjection;
+
+/// <summary>
+/// Dependency injection configuration for Application layer.
+/// </summary>
+public static class DependencyInjection
+{
+    public static IServiceCollection AddMessagingServiceApplication(this IServiceCollection services, Action<MessagingServiceApplicationConfiguration> configure)
+    {
+        var assembly = Assembly.GetExecutingAssembly();
+
+        var configuration = new MessagingServiceApplicationConfiguration(services);
+        configure(configuration);
+        configuration.EnsureConfigured();
+
+        // MediatR - Register all handlers
+        services.AddMediatR(config =>
+        {
+            config.LicenseKey = configuration.LuckyPennySoftLicenseKey;
+            config.RegisterServicesFromAssembly(assembly);
+        });
+
+        // AutoMapper - Use built-in DI extension (AutoMapper 16.2.0+)
+        services.AddAutoMapper(config =>
+        {
+            config.LicenseKey = configuration.LuckyPennySoftLicenseKey;
+            config.AddMaps(assembly);
+        });
+
+        // FluentValidation - Register all validators
+        services.AddValidatorsFromAssembly(assembly);
+
+        return services;
+    }
+
+    public static IServiceCollection AddMessagingServiceApplication(this IServiceCollection services, IConfiguration configuration)
+    {
+        return services.AddMessagingServiceApplication(cfg =>
+        {
+            cfg.SetLuckyPennySoftLicenseKey(configuration);
+            cfg.ConfigureEmailAccounts(configuration);
+        });
+    }
+}
+#endif

src/core/DigitalData.MessagingService.Application/DependencyInjection/MessagingServiceApplicationConfiguration.cs

@@ -0,0 +1,78 @@
+#if NET
+using DigitalData.MessagingService.Application.Common.Options;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace DigitalData.MessagingService.Application.DependencyInjection;
+
+public partial class MessagingServiceApplicationConfiguration
+{
+    private IServiceCollection Services { get; }
+
+    private (bool LuckyPennySoftLicenseKey, bool EmailAccounts) _configured = (LuckyPennySoftLicenseKey: false, EmailAccounts: false);
+
+    internal MessagingServiceApplicationConfiguration(IServiceCollection services)
+    {
+        Services = services;
+    }
+
+    internal void EnsureConfigured()
+    {
+        if (!_configured.LuckyPennySoftLicenseKey)
+        {
+            throw new InvalidOperationException("LuckyPennySoftLicenseKey is not configured. Please call SetLuckyPennySoftLicenseKey() or SetLuckyPennySoftLicenseKey(IConfiguration) in DI configuration.");
+        }
+        if (!_configured.EmailAccounts)
+        {
+            throw new InvalidOperationException("EmailAccountsOptions is not configured. Please call ConfigureEmailAccountsOptions() or ConfigureEmailAccounts(IConfiguration) in DI configuration.");
+        }
+    }
+
+    #region LuckyPennySoftLicenseKey
+    internal string? LuckyPennySoftLicenseKey { get; set; } = null;
+
+    public MessagingServiceApplicationConfiguration SetLuckyPennySoftLicenseKey(string licenseKey)
+    {
+        if (string.IsNullOrWhiteSpace(licenseKey))
+            throw new InvalidOperationException("LuckyPennySoftLicenseKey is not configured. Please call SetLuckyPennySoftLicenseKey() or SetLuckyPennySoftLicenseKey(IConfiguration) in DI configuration.");
+
+        LuckyPennySoftLicenseKey = licenseKey;
+        _configured.LuckyPennySoftLicenseKey = true;
+        return this;
+    }
+
+    public MessagingServiceApplicationConfiguration SetLuckyPennySoftLicenseKey(IConfiguration configuration, string key = "LuckyPennySoftLicenseKey")
+    {
+        return SetLuckyPennySoftLicenseKey(configuration.GetValue<string>(key) ?? string.Empty);
+    }
+    #endregion LuckyPennySoftLicenseKey
+
+    #region EmailAccountsOptions
+    /// <summary>
+    /// Ignores whether EmailAccountsOptions is configured or not. Use this method if you just want to publish messages to the messaging service; not to send 
+    /// emails. If you want to consume the messaging service to send emails, you must configure EmailAccountsOptions using ConfigureEmailAccountsOptions() or 
+    /// ConfigureEmailAccounts(IConfiguration).
+    /// </summary>
+    /// <returns></returns>
+    public MessagingServiceApplicationConfiguration PublishOnly()
+    {
+        _configured.EmailAccounts = true;
+        return this;
+    }
+
+    public MessagingServiceApplicationConfiguration ConfigureEmailAccountsOptions(Action<EmailAccountsOptions> configureOptions)
+    {
+        Services.Configure(configureOptions);
+        _configured.EmailAccounts = true;
+        return this;
+    }
+
+    public MessagingServiceApplicationConfiguration ConfigureEmailAccounts(IConfiguration configuration, string sectionName = EmailAccountsOptions.SectionName)
+    {
+        Services.Configure<EmailAccountsOptions>(configuration.GetSection(sectionName));
+        _configured.EmailAccounts = true;
+        return this;
+    }
+    #endregion EmailAccountsOptions
+}
+#endif