feat(application): add dependency injection configuration & documentation

- Implement DependencyInjection.cs with AddApplication() extension
  - Register MediatR with assembly scanning
  - Register pipeline behaviors in execution order:
    1. CheckQueryExecutionBehavior
    2. MainQueryExecutionBehavior
    3. ReCRequestExecutionBehavior
  - Conditional package versions (MediatR 9.0.0/12.4.1)
- Add comprehensive README.md
  - Architecture overview & CQRS pipeline flow
  - Configuration guide (DexJobOptions, ErrorAction)
  - DI setup examples (AddApplication, AddInfrastructure)
  - Exception handling patterns
  - Multi-targeting notes & troubleshooting
- Update project file with MediatR.Extensions.DependencyInjection

ECMJobRunner.Application/DependencyInjection.cs

@@ -0,0 +1,39 @@
+using ECMJobRunner.Application.Behaviors;
+using MediatR;
+using Microsoft.Extensions.DependencyInjection;
+using System.Reflection;
+
+namespace ECMJobRunner.Application
+{
+    /// <summary>
+    /// Extension methods for configuring Application layer services
+    /// </summary>
+    public static class DependencyInjection
+    {
+        /// <summary>
+        /// Adds Application layer services to the dependency injection container
+        /// Registers MediatR, pipeline behaviors, and AutoMapper
+        /// </summary>
+        /// <param name="services">The service collection</param>
+        /// <returns>The service collection for chaining</returns>
+        public static IServiceCollection AddApplication(this IServiceCollection services)
+        {
+            var assembly = Assembly.GetExecutingAssembly();
+
+            // Register MediatR with all handlers from this assembly
+#if NET48
+            services.AddMediatR(assembly);
+#else
+            services.AddMediatR(cfg => cfg.RegisterServicesFromAssembly(assembly));
+#endif
+
+            // Register pipeline behaviors in execution order
+            // Order matters: MainQuery -> CheckQuery -> ReCRequest
+            services.AddTransient(typeof(IPipelineBehavior<,>), typeof(MainQueryExecutionBehavior<,>));
+            services.AddTransient(typeof(IPipelineBehavior<,>), typeof(CheckQueryExecutionBehavior<,>));
+            services.AddTransient(typeof(IPipelineBehavior<,>), typeof(ReCRequestExecutionBehavior<,>));
+
+            return services;
+        }
+    }
+}