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.Tests/QuestionVisibilityTests.cs

@@ -0,0 +1,17 @@
+using QuestionsApi.Services;
+using Xunit;
+
+namespace QuestionsApi.Tests;
+
+public sealed class QuestionVisibilityTests
+{
+    [Theory]
+    [InlineData(true, false, true)]
+    [InlineData(true, true, false)]
+    [InlineData(false, false, false)]
+    [InlineData(false, true, false)]
+    public void IsVisibleToStudent_MatchesTruthTable(bool isPublished, bool isFlagged, bool expected)
+    {
+        Assert.Equal(expected, QuestionVisibility.IsVisibleToStudent(isPublished, isFlagged));
+    }
+}