Replace Swagger UI with Scalar

[Copilot]

Migrates API documentation from Swagger UI to Scalar for improved performance and developer experience.

Changes

• Package: Added Scalar.AspNetCore 2.11.8
• Program.cs: Replaced UseSwaggerUI with MapScalarApiReference, configured dynamic API version discovery
• ConfigureSwaggerOptions.cs: Updated terminology from "Swagger/OpenAPI" to "OpenAPI" for consistency
• README.md: Added API Documentation section

Implementation

// Before
app.UseSwagger();
app.UseSwaggerUI(options => {
    options.RoutePrefix = string.Empty;
    foreach (var description in app.DescribeApiVersions()) {
        options.SwaggerEndpoint($"/swagger/{description.GroupName}/swagger.json", 
                                description.GroupName.ToUpperInvariant());
    }
});

// After  
app.MapSwagger("/openapi/{documentName}.json");
app.MapScalarApiReference(options => {
    options.WithTitle("Best Weather Forecast API");
    var descriptions = app.DescribeApiVersions();
    foreach (var description in descriptions) {
        options.AddDocument(description.GroupName, $"/openapi/{description.GroupName}.json");
    }
});

SwaggerGen continues to generate OpenAPI documents; only the UI layer changed. All API endpoints, versioning, and existing tests remain unaffected. Scalar UI available at /scalar in development mode.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Replace Swagger UI with Scalar</issue_title>
<issue_description>Migrate the existing API documentation experience from Swagger UI to Scalar to improve performance, customization options, and long‑term maintainability. This update should preserve all current API functionality while providing a more modern, developer‑friendly interface.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Replace Swagger UI with Scalar #20

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