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/Settings/SurrealDbSettings.cs

@@ -0,0 +1,21 @@
+namespace TarninaHub.Settings;
+
+public sealed class SurrealDbSettings
+{
+    public string Namespace { get; set; } = "hubmi";
+
+    public string Database { get; set; } = "hubmi";
+
+    public string DataDirectory { get; set; } = "data";
+
+    /// <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; }
+}