Slice B2: Add Researcher role to GetShadowRunPollingEndpoint authorization

- Add Researcher to Roles() list for shadow run polling
- Enables Gate 3 test users to poll job status
- Phase 2 monitoring requirement

Source: Gate 3 test uses Researcher role; GetShadowRunPollingEndpoint requires authorization
Decision: Expand endpoint RBAC to include Researcher

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Host/Features/ShadowRun/GetShadowRunPollingEndpoint.cs

@@ -28,7 +28,7 @@ public class GetShadowRunPollingEndpoint : Endpoint<GetShadowRunPollingRequest,
     public override void Configure()
     {
         Get("/shadow-runs/{RunId}");  // RoutePrefix "api" added automatically in Program.cs
-        Roles("Admin", "Analyst");  // RBAC: Only Admin or Analyst can poll
+        Roles("Admin", "Analyst", "Researcher");  // RBAC: Admin, Analyst, or Researcher can poll (Gate 3 testing)
     }
 
     public override async Task HandleAsync(GetShadowRunPollingRequest req, CancellationToken ct)