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.

DigitalData.EmailProfiler.sln

@@ -19,7 +19,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "tests", "tests", "{4F20FEFD
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{8EC462FD-D22E-90A8-E5CE-7E832BA40C5D}"
 	ProjectSection(SolutionItems) = preProject
-		AGENTS.md = AGENTS.md
+		agents.md = agents.md
 		IMPLEMENTATION_GUIDE.md = IMPLEMENTATION_GUIDE.md
 		README.md = README.md
 		STATUS.md = STATUS.md

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
+        });
+    }
+}

src/DigitalData.EmailProfiler.API/DigitalData.EmailProfiler.API.csproj

@@ -8,6 +8,9 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Serilog.AspNetCore" Version="10.0.0" />
+    <PackageReference Include="Serilog.Sinks.Console" Version="6.1.1" />
+    <PackageReference Include="Serilog.Sinks.File" Version="7.0.0" />
     <PackageReference Include="Swashbuckle.AspNetCore" Version="6.6.2" />
 	<PackageReference Include="Microsoft.Extensions.Hosting" Version="10.0.9" />
   </ItemGroup>

src/DigitalData.EmailProfiler.API/Middleware/ExceptionHandlingMiddleware.cs

@@ -0,0 +1,100 @@
+using System.Net;
+using System.Text.Json;
+using DigitalData.EmailProfiler.Domain.Exceptions;
+
+namespace DigitalData.EmailProfiler.API.Middleware;
+
+/// <summary>
+/// Global exception handling middleware
+/// </summary>
+public class ExceptionHandlingMiddleware
+{
+    private static readonly JsonSerializerOptions _jsonSerializerOptions = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase
+    };
+
+    private readonly RequestDelegate _next;
+    private readonly ILogger<ExceptionHandlingMiddleware> _logger;
+
+    public ExceptionHandlingMiddleware(
+        RequestDelegate next,
+        ILogger<ExceptionHandlingMiddleware> logger)
+    {
+        _next = next;
+        _logger = logger;
+    }
+
+    public async Task InvokeAsync(HttpContext context)
+    {
+        try
+        {
+            await _next(context);
+        }
+        catch (Exception ex)
+        {
+            await HandleExceptionAsync(context, ex);
+        }
+    }
+
+    private async Task HandleExceptionAsync(HttpContext context, Exception exception)
+    {
+        context.Response.ContentType = "application/json";
+
+        var (statusCode, message) = exception switch
+        {
+            NotFoundException notFoundEx => 
+                (HttpStatusCode.NotFound, notFoundEx.Message),
+            
+            AuthenticationFailedException authEx => 
+                (HttpStatusCode.Unauthorized, authEx.Message),
+            
+            DmsNotAvailableException dmsEx => 
+                (HttpStatusCode.ServiceUnavailable, dmsEx.Message),
+            
+            InvalidPdfException pdfEx => 
+                (HttpStatusCode.BadRequest, pdfEx.Message),
+            
+            FluentValidation.ValidationException validationEx =>
+                (HttpStatusCode.BadRequest, FormatValidationErrors(validationEx)),
+            
+            _ => (HttpStatusCode.InternalServerError, "An internal server error occurred")
+        };
+
+        context.Response.StatusCode = (int)statusCode;
+
+        // Log the exception
+        if (statusCode == HttpStatusCode.InternalServerError)
+        {
+            _logger.LogError(exception, "Unhandled exception: {Message}", exception.Message);
+        }
+        else
+        {
+            _logger.LogWarning(exception, "Exception handled: {StatusCode} - {Message}", 
+                statusCode, message);
+        }
+
+        var response = new
+        {
+            StatusCode = (int)statusCode,
+            Message = message,
+            DetailedMessage = statusCode == HttpStatusCode.InternalServerError 
+                ? exception.Message 
+                : null,
+            Timestamp = DateTime.Now
+        };
+
+        var json = JsonSerializer.Serialize(response, _jsonSerializerOptions);
+
+        await context.Response.WriteAsync(json);
+    }
+
+    private static string FormatValidationErrors(FluentValidation.ValidationException exception)
+    {
+        var errors = exception.Errors
+            .Select(e => $"{e.PropertyName}: {e.ErrorMessage}")
+            .ToList();
+
+        return string.Join("; ", errors);
+    }
+}

src/DigitalData.EmailProfiler.API/Program.cs

@@ -1,38 +1,97 @@
 using DigitalData.EmailProfiler.API;
+using DigitalData.EmailProfiler.API.Middleware;
+using DigitalData.EmailProfiler.API.Workers;
 using DigitalData.EmailProfiler.Application;
+using DigitalData.EmailProfiler.Application.Common.Dtos;
 using DigitalData.EmailProfiler.Infrastructure;
+using Serilog;
 
-var builder = WebApplication.CreateBuilder(args);
+// Configure Serilog early
+Log.Logger = new LoggerConfiguration()
+    .WriteTo.Console()
+    .WriteTo.File(
+        path: "logs/emailprofiler-.log",
+        rollingInterval: RollingInterval.Day,
+        retainedFileCountLimit: 30,
+        outputTemplate: "{Timestamp:yyyy-MM-dd HH:mm:ss.fff zzz} [{Level:u3}] {Message:lj}{NewLine}{Exception}")
+    .CreateBootstrapLogger();
 
-// Add appsettings.Secrets.json for sensitive configuration (not committed to git)
+try
-builder.Configuration.AddJsonFile("appsettings.Secrets.json", optional: true, reloadOnChange: true);
-
-// Register Application layer (MediatR, AutoMapper, FluentValidation)
-builder.Services.AddApplicationServices();
-
-// Register Infrastructure layer (RabbitMQ, Repositories, etc.)
-builder.Services.AddInfrastructure(builder.Configuration);
-
-builder.Services.AddHostedService<Worker>();
-
-builder.Services.AddControllers();
-
-builder.Services.AddEndpointsApiExplorer();
-builder.Services.AddSwaggerGen();
-
-var app = builder.Build();
-
-// Configure the HTTP request pipeline.
-if (app.Environment.IsDevelopment())
 {
-    app.UseSwagger();
+    Log.Information("Starting EmailProfiler API");
-    app.UseSwaggerUI();
+
+    var builder = WebApplication.CreateBuilder(args);
+
+    // Use Serilog for logging
+    builder.Host.UseSerilog((context, services, configuration) => configuration
+        .ReadFrom.Configuration(context.Configuration)
+        .ReadFrom.Services(services)
+        .Enrich.FromLogContext()
+        .WriteTo.Console()
+        .WriteTo.File(
+            path: "logs/emailprofiler-.log",
+            rollingInterval: RollingInterval.Day,
+            retainedFileCountLimit: 30,
+            outputTemplate: "{Timestamp:yyyy-MM-dd HH:mm:ss.fff zzz} [{Level:u3}] {Message:lj}{NewLine}{Exception}"));
+
+    // Add appsettings.Secrets.json for sensitive configuration (not committed to git)
+    builder.Configuration.AddJsonFile("appsettings.Secrets.json", optional: true, reloadOnChange: true);
+
+    // Register Application layer (MediatR, AutoMapper, FluentValidation)
+    builder.Services.AddApplicationServices();
+
+    // Register Infrastructure layer (RabbitMQ, Repositories, etc.)
+    builder.Services.AddInfrastructure(builder.Configuration);
+
+    // Register EmailSenderWorker configuration
+    builder.Services.Configure<EmailSenderWorkerConfiguration>(
+        builder.Configuration.GetSection(EmailSenderWorkerConfiguration.SectionName));
+
+    // Register EmailAccount configuration (IOptions<EmailAccountDto>)
+    builder.Services.Configure<EmailAccountDto>(
+        builder.Configuration.GetSection("EmailAccount"));
+
+    // Register Background Workers
+    builder.Services.AddHostedService<Worker>();
+    builder.Services.AddHostedService<EmailSenderWorker>();
+
+    builder.Services.AddControllers();
+
+    builder.Services.AddEndpointsApiExplorer();
+    builder.Services.AddSwaggerGen();
+
+    var app = builder.Build();
+
+    // Add global exception handling middleware
+    app.UseMiddleware<ExceptionHandlingMiddleware>();
+
+    // Add Serilog request logging
+    app.UseSerilogRequestLogging();
+
+    // Configure the HTTP request pipeline.
+    if (app.Environment.IsDevelopment())
+    {
+        app.UseSwagger();
+        app.UseSwaggerUI();
+    }
+
+    app.UseHttpsRedirection();
+
+    app.UseAuthorization();
+
+    app.MapControllers();
+
+    Log.Information("EmailProfiler API started successfully");
+
+    app.Run();
+}
+catch (Exception ex)
+{
+    Log.Fatal(ex, "EmailProfiler API failed to start");
+    throw;
+}
+finally
+{
+    Log.CloseAndFlush();
 }
 
-app.UseHttpsRedirection();
-
-app.UseAuthorization();
-
-app.MapControllers();
-
-app.Run();

src/DigitalData.EmailProfiler.API/appsettings.json

@@ -1,9 +1,28 @@
 {
-  "Logging": {
+  "Serilog": {
-    "LogLevel": {
+    "MinimumLevel": {
       "Default": "Information",
-      "Microsoft.AspNetCore": "Warning"
+      "Override": {
+        "Microsoft": "Warning",
+        "Microsoft.AspNetCore": "Warning",
+        "Microsoft.EntityFrameworkCore": "Warning",
+        "System": "Warning"
+      }
     }
   },
-  "AllowedHosts": "*"
+  "AllowedHosts": "*",
-}
+  "Workers": {
+    "EmailSender": {
+      "Enabled": true,
+      "MaxRetryCount": 3
+    }
+  },
+  "EmailAccount": {
+    "Username": "test-flow@digitaldata.works",
+    "SmtpServer": "kundencenter.triplew.de",
+    "SmtpPort": 465,
+    "SmtpUseSsl": true,
+    "UseOAuth2": false
+  },
+  "LuckyPennySoftLicenseKey": "eyJhbGciOiJSUzI1NiIsImtpZCI6Ikx1Y2t5UGVubnlTb2Z0d2FyZUxpY2Vuc2VLZXkvYmJiMTNhY2I1OTkwNGQ4OWI0Y2IxYzg1ZjA4OGNjZjkiLCJ0eXAiOiJKV1QifQ.eyJpc3MiOiJodHRwczovL2x1Y2t5cGVubnlzb2Z0d2FyZS5jb20iLCJhdWQiOiJMdWNreVBlbm55U29mdHdhcmUiLCJleHAiOiIxODE2MTI4MDAwIiwiaWF0IjoiMTc4NDYyNDU1NyIsImFjY291bnRfaWQiOiIwMTk4M2M1OWU0YjM3MjhlYmZkMzEwM2MyYTQ4NmU4NSIsImN1c3RvbWVyX2lkIjoiMDE5ODNjNTllNGIzNzI4ZWJmZDMxMDNjMmE0ODZlODUiLCJzdWJfaWQiOiItIiwiZWRpdGlvbiI6IjAiLCJ0eXBlIjoiMiJ9.IUUO926m9crYGYxMjjKD_n9BnUm-EDyjFIn0YmMUCo7C-QTwvB8WhXP8veTSFsBq-leIIDJ4jyl7Pgc_7ciwg1XhUSIs4mkQroEUaSFCGOxw7Pi41WM8MK5YFSaqLTYYXec9zxgiJbGzABbh3CHTSup3okGnVm_CMoPEs91l2c0A6N1JyZy74urd_tF0KGVKf0MOvzdlQIWLQ8o73S4pTv2N-F6UlzI0fdMtTHMLNNQyr0NdWdnuBk_jMBXO-gy5RE_oCRfMTTYRX2n3XLK6pTfXE0Ct338o9F5sH8Ph2lTXSu56cpdsfZOQZGqCH0LoFp1Dd7RJgIgNmBiTGfvDnA"
+}

src/DigitalData.EmailProfiler.Domain/Common/DomainConstants.cs

@@ -0,0 +1,19 @@
+namespace DigitalData.EmailProfiler.Domain.Common;
+
+/// <summary>
+/// Domain-wide constants
+/// </summary>
+public static class DomainConstants
+{
+    /// <summary>
+    /// Email processing constants
+    /// </summary>
+    public static class Email
+    {
+        /// <summary>
+        /// Maximum number of retry attempts for failed email sending
+        /// After this limit, email will be moved to Dead Letter Queue (DLQ)
+        /// </summary>
+        public const int MaxRetryCount = 3;
+    }
+}