feat: implement Exchange Rate REST API with CNB provider

jobs/Backend/Task/ExchangeRateUpdater.sln

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jobs/Backend/Task/README.md

@@ -0,0 +1,94 @@
+# Exchange Rate API
+
+A REST API service that provides daily currency exchange rates from the Czech National Bank. Built with clean architecture principles, the service fetches, caches, and serves exchange rate data with support for filtering by currency and date.
+
+**Purpose:** Simplify currency conversion by providing a reliable, fast, and well-documented API for accessing real-time and historical exchange rates against the Czech Koruna (CZK).
+
+## Tech Stack
+
+- **.NET 10.0** - Latest cross-platform framework
+- **ASP.NET Core** - Web API
+- **DataAnnotation** - Request validation
+- **Microsoft.Extensions.Http.Resilience** - HTTP resilience (retry, circuit breaker, timeout)
+- **Scalar** - Interactive API documentation
+- **xUnit v3** - Unit testing
+- **NSubstitute** - Mocking
+
+## Quick Start
+
+### Prerequisites
+
+- [.NET 10.0 SDK](https://dotnet.microsoft.com/download)
+
+### Run the API
+
+```bash
+cd jobs/Backend/Task
+dotnet run --project src/ExchangeRateUpdater.Api
+```
+
+Access the API at:
+- **API Docs**: http://localhost:5213/scalar/v1
+- **Swagger**: http://localhost:5213/openapi/v1.json
+
+### Run Tests
+
+```bash
+dotnet test
+```
+
+All tests should pass.
+
+### Response Format
+
+**Success (200 OK):**
+```json
+{
+  "data": [
+    {
+      "sourceCurrency": "USD",
+      "targetCurrency": "CZK",
+      "rate": 23.45
+    }
+  ]
+}
+```
+
+**Error (400/404/502):**
+```json
+{
+  "errorCode": "ExchangeRate.CurrencyNotFound",
+  "errorMessage": "Exchange rate for currency 'XYZ' was not found in the source data."
+}
+```
+
+## Architecture
+
+Clean architecture with 4 layers:
+
+```
+Domain/              # Entities, value objects (Currency, ExchangeRate)
+Application/         # Business logic, caching decorator
+Infrastructure/      # External APIs, HTTP clients, CNB provider
+Presentation/        # Controllers, validation, API responses
+```
+
+## Adding More Providers
+
+To add a new exchange rate provider (e.g., ECB, Bank of England):
+
+1. **Create provider folder:** `Infrastructure/Providers/Ecb/`
+2. **Implement HTTP client:** Create `EcbApiClient` with provider-specific logic
+3. **Create adapter:** Implement `IExternalExchangeRateClient` to map to common format
+4. **Register in DI:** Add to `Infrastructure/Extensions/ServiceCollectionExtensions.cs`
+5. **Configure:** Add provider settings to `appsettings.json`
+
+**Run with hot reload:**
+```bash
+dotnet watch --project src/ExchangeRateUpdater.Api
+```
+
+**Test with coverage:**
+```bash
+dotnet test --collect:"XPlat Code Coverage"
+```

jobs/Backend/Task/nuget.config

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
+  </packageSources>
+</configuration>

jobs/Backend/Task/src/ExchangeRateUpdater.Api/Application/Extensions/ServiceCollectionExtensions.cs

@@ -0,0 +1,30 @@
+using ExchangeRateUpdater.Api.Application.Interfaces;
+using ExchangeRateUpdater.Api.Application.Services;
+using ExchangeRateUpdater.Api.Infrastructure.Providers.Cnb;
+using Microsoft.Extensions.Caching.Memory;
+using Microsoft.Extensions.Options;
+
+namespace ExchangeRateUpdater.Api.Application.Extensions;
+
+public static class ServiceCollectionExtensions
+{
+    public static IServiceCollection AddApplication(this IServiceCollection services)
+    {
+        services.AddScoped<ExchangeRateProvider>();
+
+        services.AddScoped<IExchangeRateProvider>(sp =>
+        {
+            var inner = sp.GetRequiredService<ExchangeRateProvider>();
+            var cache = sp.GetRequiredService<IMemoryCache>();
+            var logger = sp.GetRequiredService<ILogger<ExchangeRateProviderDecorator>>();
+            var options = sp.GetRequiredService<IOptions<CnbApiOptions>>().Value;
+
+            return new ExchangeRateProviderDecorator(
+                inner, cache, logger,
+                TimeSpan.FromMinutes(options.CacheDurationMinutes),
+                TimeSpan.FromMinutes(options.HistoricalCacheDurationMinutes));
+        });
+
+        return services;
+    }
+}

jobs/Backend/Task/src/ExchangeRateUpdater.Api/Application/Interfaces/IExchangeRateProvider.cs

@@ -0,0 +1,11 @@
+using ExchangeRateUpdater.Api.Domain;
+using ExchangeRateUpdater.Api.Domain.Common;
+
+namespace ExchangeRateUpdater.Api.Application.Interfaces;
+
+public interface IExchangeRateProvider
+{
+    Task<Result<IReadOnlyCollection<ExchangeRate>>> GetDailyRatesAsync(
+        DateOnly? date = null,
+        CancellationToken cancellationToken = default);
+}

jobs/Backend/Task/src/ExchangeRateUpdater.Api/Application/Services/ExchangeRateProvider.cs

@@ -0,0 +1,50 @@
+using ExchangeRateUpdater.Api.Application.Interfaces;
+using ExchangeRateUpdater.Api.Domain;
+using ExchangeRateUpdater.Api.Domain.Common;
+using ExchangeRateUpdater.Api.Infrastructure.Interfaces;
+using ExchangeRateUpdater.Api.Infrastructure.Providers.Cnb.Models;
+
+namespace ExchangeRateUpdater.Api.Application.Services;
+
+public sealed class ExchangeRateProvider(
+    IExternalExchangeRateClient externalClient,
+    ILogger<ExchangeRateProvider> logger) : IExchangeRateProvider
+{
+    private const string TargetCurrencyCode = "CZK";
+
+    public async Task<Result<IReadOnlyCollection<ExchangeRate>>> GetDailyRatesAsync(
+        DateOnly? date = null,
+        CancellationToken cancellationToken = default)
+    {
+        var externalResult = await externalClient.GetDailyRatesAsync(date, cancellationToken);
+
+        if (externalResult.IsFailure)
+            return Result<IReadOnlyCollection<ExchangeRate>>.Failure(externalResult.Error);
+
+        var domainRates = new List<ExchangeRate>();
+
+        foreach (var external in externalResult.Value.Rates)
+        {
+            var result = MapToDomainModel(external);
+            if (result.IsFailure)
+                return Result<IReadOnlyCollection<ExchangeRate>>.Failure(result.Error);
+
+            domainRates.Add(result.Value);
+        }
+
+        logger.LogInformation(
+            "Mapped {Count} exchange rates for date {Date}",
+            domainRates.Count,
+            date?.ToString("yyyy-MM-dd") ?? "latest");
+
+        return Result<IReadOnlyCollection<ExchangeRate>>.Success(domainRates.ToArray());
+    }
+
+    private static Result<ExchangeRate> MapToDomainModel(CnbExRateDaily external) =>
+        ExchangeRate.Create(
+            external.CurrencyCode,
+            TargetCurrencyCode,
+            external.Rate,
+            external.Amount,
+            DateOnly.Parse(external.ValidFor));
+}