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/Services/StorageService.cs

@@ -0,0 +1,57 @@
+using Microsoft.Extensions.Options;
+using SurrealDb.Net;
+using TarninaHub.Models;
+using TarninaHub.Settings;
+
+namespace TarninaHub.Services;
+
+public sealed class StorageService : IStorageService
+{
+    private const string ProbeTable = "storage_probe";
+
+    private readonly ISurrealDbClient _client;
+    private readonly SurrealDbSettings _settings;
+    private readonly string _dataDirectory;
+
+    public StorageService(ISurrealDbClient client, IOptions<SurrealDbSettings> settings)
+    {
+        _client = client;
+        _settings = settings.Value;
+        _dataDirectory = Path.Combine(AppContext.BaseDirectory, _settings.DataDirectory);
+    }
+
+    public async Task InitializeAsync(CancellationToken cancellationToken)
+    {
+        await _client.RawQuery(
+            $"DEFINE TABLE IF NOT EXISTS {ProbeTable} SCHEMALESS;",
+            cancellationToken: cancellationToken);
+    }
+
+    public StorageInfo GetInfo()
+    {
+        var isRemote = !string.IsNullOrWhiteSpace(_settings.RemoteUrl);
+
+        return new StorageInfo(
+            isRemote ? "remote" : "rocksdb",
+            isRemote ? _settings.RemoteUrl! : _dataDirectory,
+            _settings.Namespace,
+            _settings.Database);
+    }
+
+    public async Task<int> GetProbeCountAsync(CancellationToken cancellationToken)
+    {
+        var probes = await _client.Select<StorageProbe>(ProbeTable, cancellationToken);
+
+        return probes.Count();
+    }
+
+    public Task<StorageProbe> AddProbeAsync(CancellationToken cancellationToken)
+    {
+        var probe = new StorageProbe
+        {
+            CreatedAt = DateTimeOffset.UtcNow,
+        };
+
+        return _client.Create(ProbeTable, probe, cancellationToken);
+    }
+}