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/Controllers/SystemController.cs

@@ -0,0 +1,39 @@
+using Microsoft.AspNetCore.Mvc;
+using TarninaHub.Models;
+using TarninaHub.Services;
+
+namespace TarninaHub.Controllers;
+
+[ApiController]
+[Route("api/system")]
+public class SystemController : ControllerBase
+{
+    private readonly IStorageService _storageService;
+
+    public SystemController(IStorageService storageService)
+    {
+        _storageService = storageService;
+    }
+
+    [HttpGet("storage")]
+    public async Task<ActionResult<StorageStatusResponse>> GetStorage(CancellationToken cancellationToken)
+    {
+        var info = _storageService.GetInfo();
+        var probeCount = await _storageService.GetProbeCountAsync(cancellationToken);
+
+        return Ok(new StorageStatusResponse(
+            info.Engine,
+            info.DataDirectory,
+            info.Namespace,
+            info.Database,
+            probeCount));
+    }
+
+    [HttpPost("storage/probes")]
+    public async Task<ActionResult<StorageProbeResponse>> CreateProbe(CancellationToken cancellationToken)
+    {
+        var probe = await _storageService.AddProbeAsync(cancellationToken);
+
+        return Ok(new StorageProbeResponse(probe.Label, probe.CreatedAt));
+    }
+}