Add a review-flag state for questions, independent of publish status

Flagged questions disappear from every student-facing surface (search,
exam simulation sampling, the missed-question review queue, bookmarks,
direct fetch) while staying published - flagging and unpublishing are
now separate, reversible actions with their own audit trail (who
flagged it, when, why). Backend-only for now: SUPER-only flag/unflag/
list endpoints, no client UI. Includes a one-time CLI sweep
(flag-media-questions) to flag every existing question with an image
so they can be reviewed.

api/QuestionsApi/Models/QuestionRequests.cs

@@ -38,6 +38,11 @@ public class UpsertQuestionRequest
     public bool IsPublished { get; set; } = true;
 }
 
+public class FlagQuestionRequest
+{
+    public string Reason { get; set; } = "";
+}
+
 public class FirstQuestionAttemptRequest
 {
     public string QuestionExternalId { get; set; } = "";