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/Application/Abstractions/IMigrationRunner.cs

@@ -0,0 +1,6 @@
+namespace TarninaHub.Application.Abstractions;
+
+public interface IMigrationRunner
+{
+    Task ApplyAsync(CancellationToken cancellationToken);
+}

backend/Application/Abstractions/IStorageService.cs

@@ -0,0 +1,14 @@
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Application.Abstractions;
+
+public interface IStorageService
+{
+    Task InitializeAsync(CancellationToken cancellationToken);
+
+    StorageInfo GetInfo();
+
+    Task<int> GetProbeCountAsync(CancellationToken cancellationToken);
+
+    Task<StorageProbe> AddProbeAsync(CancellationToken cancellationToken);
+}