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/Errors/GlobalExceptionHandler.cs

@@ -0,0 +1,38 @@
+using Microsoft.AspNetCore.Diagnostics;
+using Microsoft.AspNetCore.Mvc;
+
+namespace TarninaHub.Errors;
+
+public sealed class GlobalExceptionHandler : IExceptionHandler
+{
+    private readonly ILogger<GlobalExceptionHandler> _logger;
+
+    public GlobalExceptionHandler(ILogger<GlobalExceptionHandler> logger)
+    {
+        _logger = logger;
+    }
+
+    public async ValueTask<bool> TryHandleAsync(
+        HttpContext httpContext,
+        Exception exception,
+        CancellationToken cancellationToken)
+    {
+        _logger.LogError(exception, "Unhandled exception for {Method} {Path}",
+            httpContext.Request.Method,
+            httpContext.Request.Path);
+
+        var problem = new ProblemDetails
+        {
+            Type = "https://httpstatuses.io/500",
+            Title = "An unexpected error occurred.",
+            Status = StatusCodes.Status500InternalServerError,
+            Instance = httpContext.Request.Path,
+        };
+
+        httpContext.Response.StatusCode = problem.Status.Value;
+
+        await httpContext.Response.WriteAsJsonAsync(problem, cancellationToken);
+
+        return true;
+    }
+}