feat(tutorial): add /tutorial/update — echo step + persist to viewer

POST /tutorial/update echoes tutorial_step back and saves it to
Viewer.MissionData.TutorialState. is_skip=1 is handled server-side
by honoring whatever tutorial_step value the client sends (client
already sends 100 when skipping). Adds TutorialUpdateRequest DTO,
TutorialUpdateResponse DTO, injects SVSimDbContext into
TutorialController, and adds GetViewerTutorialStateAsync helper to
SVSimTestFactory.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.UnitTests/Infrastructure/SVSimTestFactory.cs

@@ -372,6 +372,18 @@ internal sealed class SVSimTestFactory : WebApplicationFactory<Program>
         await db.SaveChangesAsync();
     }
 
+    /// <summary>
+    /// Reads the viewer's current <c>TutorialState</c> from the DB.
+    /// Tests use this to verify that <c>/tutorial/update</c> persisted the step.
+    /// </summary>
+    public async Task<int> GetViewerTutorialStateAsync(long viewerId)
+    {
+        using var scope = Services.CreateScope();
+        var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+        var viewer = await db.Viewers.Include(v => v.MissionData).FirstAsync(v => v.Id == viewerId);
+        return viewer.MissionData.TutorialState;
+    }
+
     protected override void Dispose(bool disposing)
     {
         base.Dispose(disposing);