feat: Add domain constants, API infrastructure, and configuration

Domain Layer:
- Add DomainConstants for email, attachment, and process constants

API Layer:
- Add EmailsController (minimal REST API endpoints)
- Add ExceptionHandlingMiddleware for global exception handling
- Update Program.cs:
  * Add EmailProfilerDbContext registration (SQL Server)
  * Add Generic Repository<T> scoped registration
  * Add ExceptionHandlingMiddleware to pipeline
  * Add EmailSenderWorker as hosted service
  * Configure Serilog file logging
  * Add Scalar OpenAPI documentation

Configuration:
- Add EmailAccount section in appsettings.json (SMTP credentials)
- Add RabbitMq section (message queue configuration)
- Add Serilog file sink configuration
- Update .csproj with required NuGet packages
- Update solution file

This commit completes the basic API infrastructure setup.

src/DigitalData.EmailProfiler.API/Controllers/EmailsController.cs

@@ -0,0 +1,37 @@
+using DigitalData.EmailProfiler.Application.EmailSending.Commands;
+using MediatR;
+using Microsoft.AspNetCore.Mvc;
+
+namespace DigitalData.EmailProfiler.API.Controllers;
+
+/// <summary>
+/// Email sending API controller
+/// Enqueues outgoing emails to RabbitMQ for async processing
+/// </summary>
+[ApiController]
+[Route("api/[controller]")]
+public class EmailsController(IMediator mediator) : ControllerBase
+{
+    /// <summary>
+    /// Send email (enqueue to RabbitMQ for background processing)
+    /// </summary>
+    /// <param name="command">Send email command</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>HTTP 202 Accepted (queued for processing)</returns>
+    [HttpPost("send")]
+    [ProducesResponseType(StatusCodes.Status202Accepted)]
+    [ProducesResponseType(StatusCodes.Status400BadRequest)]
+    public async Task<IActionResult> SendEmail([FromBody] SendEmailCommand command, CancellationToken cancellationToken)
+    {
+        var emailOutboxId = await mediator.Send(command, cancellationToken);
+
+        return Accepted(new
+        {
+            Message = "Email queued for sending",
+            EmailOutboxId = emailOutboxId,
+            To = command.Recipient,
+            command.Subject,
+            QueuedAt = DateTime.Now
+        });
+    }
+}