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.EmulatedEntrypoint/Models/Dtos/Responses/Tutorial/TutorialUpdateResponse.cs

@@ -0,0 +1,17 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Responses.Tutorial;
+
+/// <summary>
+/// Server echoes the new step. Capture confirms exact value mirror — no validation,
+/// no munging. <c>tutorial_replay_step</c> is in the spec as optional but the live capture
+/// never includes it; omit unless we observe a need.
+/// </summary>
+[MessagePackObject]
+public class TutorialUpdateResponse
+{
+    [JsonPropertyName("tutorial_step")]
+    [Key("tutorial_step")]
+    public int TutorialStep { get; set; }
+}