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/Question.cs

@@ -30,6 +30,9 @@ public class Question
     public List<string> Tags { get; set; } = [];
     public List<QuestionReference> References { get; set; } = [];
     public bool IsPublished { get; set; } = true;
+    public DateTime? FlaggedForReviewUtc { get; set; }
+    public string FlagReason { get; set; } = "";
+    public string FlaggedByUserId { get; set; } = "";
     public DateTime CreatedUtc { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedUtc { get; set; } = DateTime.UtcNow;
 }