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);
+}

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);

backend/Application/TarninaHub.Application.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Domain\TarninaHub.Domain.csproj" />
+  </ItemGroup>
+
+</Project>