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/Dtos/StorageModels.cs

@@ -0,0 +1,25 @@
+namespace TarninaHub.Application.Dtos;
+
+public sealed class StorageProbe
+{
+    public string Label { get; set; } = "probe";
+
+    public DateTimeOffset CreatedAt { get; set; }
+}
+
+public sealed record StorageInfo(
+    string Engine,
+    string DataDirectory,
+    string Namespace,
+    string Database);
+
+public sealed record StorageStatusResponse(
+    string Engine,
+    string DataDirectory,
+    string Namespace,
+    string Database,
+    int ProbeCount);
+
+public sealed record StorageProbeResponse(
+    string Label,
+    DateTimeOffset CreatedAt);