feat(backend): restructure API into layers with OpenAPI, versioning and error handling

Split the backend into API host, Domain, Application and Infrastructure projects. Add Swagger/OpenAPI per API version, Asp.Versioning (query/header), ProblemDetails with a global exception handler and SurrealQL migrations applied at startup (namespace/database ensured). Infrastructure owns the SurrealDB client, settings and DI.

backend/Configuration/ConfigureSwaggerOptions.cs

@@ -0,0 +1,29 @@
+using Asp.Versioning.ApiExplorer;
+using Microsoft.Extensions.Options;
+using Microsoft.OpenApi;
+using Swashbuckle.AspNetCore.SwaggerGen;
+
+namespace TarninaHub.Configuration;
+
+public sealed class ConfigureSwaggerOptions : IConfigureOptions<SwaggerGenOptions>
+{
+    private readonly IApiVersionDescriptionProvider _provider;
+
+    public ConfigureSwaggerOptions(IApiVersionDescriptionProvider provider)
+    {
+        _provider = provider;
+    }
+
+    public void Configure(SwaggerGenOptions options)
+    {
+        foreach (var description in _provider.ApiVersionDescriptions)
+        {
+            options.SwaggerDoc(description.GroupName, new OpenApiInfo
+            {
+                Title = "TarninaHub API",
+                Version = description.ApiVersion.ToString(),
+                Description = "Backend API (BFF) for TarninaHub — Małopolski Hub Innowacji Społecznych.",
+            });
+        }
+    }
+}