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/Infrastructure/Settings/SurrealDbEndpoint.cs

@@ -0,0 +1,21 @@
+namespace TarninaHub.Infrastructure.Settings;
+
+public static class SurrealDbEndpoint
+{
+    /// <summary>
+    /// Builds the SurrealDB endpoint. Remote mode uses the configured URL;
+    /// embedded mode uses the RocksDB provider persisted to the data directory
+    /// (forward slashes, so the same endpoint works on Windows and Linux).
+    /// </summary>
+    public static string Build(SurrealDbSettings settings)
+    {
+        if (!string.IsNullOrWhiteSpace(settings.RemoteUrl))
+        {
+            return settings.RemoteUrl;
+        }
+
+        var dataDirectory = Path.Combine(AppContext.BaseDirectory, settings.DataDirectory);
+
+        return $"rocksdb://{dataDirectory.Replace('\\', '/')}";
+    }
+}

backend/Infrastructure/Settings/SurrealDbSettings.cs

@@ -0,0 +1,27 @@
+namespace TarninaHub.Infrastructure.Settings;
+
+public sealed class SurrealDbSettings
+{
+    public string Namespace { get; set; } = "hubmi";
+
+    public string Database { get; set; } = "hubmi";
+
+    public string DataDirectory { get; set; } = "data";
+
+    /// <summary>
+    /// Directory (relative to the app base directory) with ordered *.surql
+    /// migration files applied at startup. Copied from backend/migrations.
+    /// </summary>
+    public string MigrationsDirectory { get; set; } = "migrations";
+
+    /// <summary>
+    /// When set (e.g. ws://127.0.0.1:8010/rpc), the backend connects to a
+    /// dedicated SurrealDB server instead of the embedded RocksDB engine.
+    /// The server must persist to ./data, not to memory.
+    /// </summary>
+    public string? RemoteUrl { get; set; }
+
+    public string? Username { get; set; }
+
+    public string? Password { get; set; }
+}