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/Program.cs

@@ -1,12 +1,55 @@
+using TarninaHub.Services;
+using TarninaHub.Settings;
+
 var builder = WebApplication.CreateBuilder(args);
 
+var surrealSettings = builder.Configuration.GetSection("SurrealDb").Get<SurrealDbSettings>()
+    ?? new SurrealDbSettings();
+
+builder.Services.Configure<SurrealDbSettings>(builder.Configuration.GetSection("SurrealDb"));
+
 // Learn more about configuring OpenAPI at https://aka.ms/aspnet/openapi
 builder.Services.AddOpenApi();
 
 builder.Services.AddControllers();
 
+builder.Services.AddSingleton<IStorageService, StorageService>();
+
+var surrealOptions = new SurrealDbOptions
+{
+    Endpoint = BuildEndpoint(surrealSettings),
+    Namespace = surrealSettings.Namespace,
+    Database = surrealSettings.Database,
+};
+
+if (string.IsNullOrWhiteSpace(surrealSettings.RemoteUrl))
+{
+    // Embedded RocksDB, persisted next to the running app (./data), never in memory.
+    var dataDirectory = Path.Combine(AppContext.BaseDirectory, surrealSettings.DataDirectory);
+    Directory.CreateDirectory(dataDirectory);
+
+    builder.Services
+        .AddSurreal(surrealOptions, ServiceLifetime.Singleton)
+        .AddRocksDbProvider();
+}
+else
+{
+    // Remote SurrealDB server on its own dedicated port (used for sync with the frontend).
+    surrealOptions.Username = surrealSettings.Username;
+    surrealOptions.Password = surrealSettings.Password;
+
+    builder.Services.AddSurreal(surrealOptions, ServiceLifetime.Singleton);
+}
+
 var app = builder.Build();
 
+using (var scope = app.Services.CreateScope())
+{
+    var storageService = scope.ServiceProvider.GetRequiredService<IStorageService>();
+
+    await storageService.InitializeAsync(CancellationToken.None);
+}
+
 // Configure the HTTP request pipeline.
 if (app.Environment.IsDevelopment())
 {
@@ -25,3 +68,15 @@ app.MapControllers();
 app.MapFallbackToFile("index.html");
 
 app.Run();
+
+static string BuildEndpoint(SurrealDbSettings settings)
+{
+    if (!string.IsNullOrWhiteSpace(settings.RemoteUrl))
+    {
+        return settings.RemoteUrl;
+    }
+
+    var dataDirectory = Path.Combine(AppContext.BaseDirectory, settings.DataDirectory);
+
+    return $"rocksdb://{dataDirectory.Replace('\\', '/')}";
+}