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/Domain/Enums/PartnershipStatus.cs

@@ -0,0 +1,8 @@
+namespace TarninaHub.Domain.Enums;
+
+public enum PartnershipStatus
+{
+    Proposed,
+    Active,
+    Closed,
+}