feat(sleeve): /sleeve/favorite accept-only (persistence deferred)

SVSim.EmulatedEntrypoint/Controllers/SleeveController.cs

@@ -169,6 +169,16 @@ public class SleeveController : SVSimController
             .ToList();
     }
 
+    [HttpPost("favorite")]
+    public ActionResult<EmptyResponse> Favorite([FromBody] SleeveFavoriteRequest _)
+    {
+        if (!TryGetViewerId(out long __)) return Unauthorized();
+        // Accept-and-ack. Persisting per-viewer cosmetic favorites is deferred until
+        // a viewer-favorites schema lands (would also serve /emblem/favorite + /degree/favorite
+        // via the shared Wizard/FavoriteTask.cs Kind enum). Same pattern as ConfigController.
+        return new EmptyResponse();
+    }
+
     /// <summary>
     /// A product is "purchased" once the viewer owns at least one of its sleeve-typed reward
     /// grants. Emblem/other grants aren't load-bearing for this check — a viewer who somehow