feat(backend): integrate SurrealDB storage with embedded RocksDB

Add StorageService backed by SurrealDB using the embedded RocksDB provider that persists to ./data (never in memory). Support an optional remote server via SurrealDb:RemoteUrl for frontend sync. Expose readiness and storage probe endpoints under /api/system and add a helper script to start a local SurrealDB server.

Set the backend version to 0.0.1.

backend/Models/StorageModels.cs

@@ -0,0 +1,25 @@
+namespace TarninaHub.Models;
+
+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);