scaffold: add messaging lib and users-service skeleton with tests

Backend/common/messaging/IKafkaProducer.cs

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+using System.Threading;
+using System.Threading.Tasks;
+
+namespace ExpensesManager.Common.Messaging
+{
+    public interface IKafkaProducer
+    {
+        Task ProduceAsync(string topic, object message, CancellationToken cancellationToken = default);
+    }
+}

Backend/common/messaging/KafkaProducer.cs

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+using System.Text.Json;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.Logging;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace ExpensesManager.Common.Messaging
+{
+    public class KafkaProducer : IKafkaProducer
+    {
+        private readonly ILogger<KafkaProducer> _logger;
+        private readonly string _brokers;
+
+        public KafkaProducer(ILogger<KafkaProducer> logger, IConfiguration config)
+        {
+            _logger = logger;
+            _brokers = config["KAFKA_BROKERS"] ?? "localhost:9092";
+        }
+
+        public Task ProduceAsync(string topic, object message, CancellationToken cancellationToken = default)
+        {
+            var json = JsonSerializer.Serialize(message);
+            _logger.LogInformation("[KafkaProducer] ProduceAsync -> brokers={Brokers} topic={Topic} message={Message}", _brokers, topic, json);
+            // No-op implementation for now (dev-friendly). Replace with real Kafka client when ready.
+            return Task.CompletedTask;
+        }
+    }
+}

Backend/common/messaging/Messaging.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net7.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" Version="7.0.0" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.Abstractions" Version="7.0.0" />
+  </ItemGroup>
+</Project>

Backend/services/users-service/Controllers/UsersController.cs

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+using System.Threading.Tasks;
+using ExpensesManager.Common.Messaging;
+using Microsoft.AspNetCore.Mvc;
+
+namespace ExpensesManager.UsersService.Controllers
+{
+    [ApiController]
+    [Route("api/[controller]")]
+    public class UsersController : ControllerBase
+    {
+        private readonly IKafkaProducer _producer;
+
+        public UsersController(IKafkaProducer producer)
+        {
+            _producer = producer;
+        }
+
+        [HttpGet("health")]
+        public IActionResult Health() => Ok(new { status = "ok" });
+
+        public record UserDto(int UserId, string Username, string? Email);
+
+        [HttpPost("signup")]
+        public async Task<IActionResult> SignUp([FromBody] UserDto dto)
+        {
+            var ev = new { eventType = "UserCreated", payload = dto };
+            await _producer.ProduceAsync("UserCreated", ev);
+            return Created(string.Empty, dto);
+        }
+    }
+}

Backend/services/users-service/Program.cs

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+using ExpensesManager.Common.Messaging;
+
+var builder = WebApplication.CreateBuilder(args);
+
+builder.Services.AddControllers();
+builder.Services.AddSingleton<IKafkaProducer, KafkaProducer>();
+builder.Services.AddEndpointsApiExplorer();
+builder.Services.AddSwaggerGen();
+
+var app = builder.Build();
+
+if (app.Environment.IsDevelopment())
+{
+    app.UseSwagger();
+    app.UseSwaggerUI();
+}
+
+app.MapControllers();
+
+app.Run();

Backend/services/users-service/UsersService.csproj

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+<Project Sdk="Microsoft.NET.Sdk.Web">
+  <PropertyGroup>
+    <TargetFramework>net7.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\common\messaging\Messaging.csproj" />
+  </ItemGroup>
+  <ItemGroup>
+    <!-- Exclude any test files located under the tests folder so they aren't compiled into the web project -->
+    <Compile Remove="tests\**\*.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <PackageReference Include="Swashbuckle.AspNetCore" Version="6.5.0" />
+  </ItemGroup>
+</Project>

Backend/services/users-service/tests/UsersService.Tests/UsersControllerTests.cs

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+using System.Threading;
+using System.Threading.Tasks;
+using ExpensesManager.Common.Messaging;
+using ExpensesManager.UsersService.Controllers;
+using Microsoft.AspNetCore.Mvc;
+using Xunit;
+
+namespace UsersService.Tests
+{
+    class RecordingProducer : IKafkaProducer
+    {
+        public bool WasCalled { get; private set; }
+        public Task ProduceAsync(string topic, object message, CancellationToken cancellationToken = default)
+        {
+            WasCalled = true;
+            return Task.CompletedTask;
+        }
+    }
+
+    public class UsersControllerTests
+    {
+        [Fact]
+        public void Health_returns_ok()
+        {
+            var controller = new UsersController(new RecordingProducer());
+            var result = controller.Health();
+            var ok = Assert.IsType<OkObjectResult>(result);
+            Assert.Equal(200, ok.StatusCode);
+        }
+
+        [Fact]
+        public async Task SignUp_returns_created_and_calls_producer()
+        {
+            var producer = new RecordingProducer();
+            var controller = new UsersController(producer);
+            var dto = new UsersController.UserDto(1, "alice", "a@a.com");
+            var res = await controller.SignUp(dto);
+            var created = Assert.IsType<CreatedResult>(res);
+            Assert.Equal(dto, created.Value);
+            Assert.True(producer.WasCalled);
+        }
+    }
+}

Backend/services/users-service/tests/UsersService.Tests/UsersService.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net7.0</TargetFramework>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="xunit" Version="2.4.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\UsersService.csproj" />
+    <ProjectReference Include="..\..\..\..\common\messaging\Messaging.csproj" />
+  </ItemGroup>
+</Project>