Backgrounds: one locked Background per screen + mini rule (TASK 25)

Services/LayoutStore.cs

@@ -390,7 +390,7 @@ public class LayoutStore : IDisposable
     // v5 → v6: Scene gains HasBackground. Added columns default to true; the
     // ONE-TIME backfill (runs only when the column is first added) turns every
     // non-Live canonical scene off and drops their background sources — the fix for
-    // DBs saved before the Live-only policy. MainViewModel.EnforceBackgroundPolicy
+    // DBs saved before the Live-only policy. MainViewModel.NormalizeBackgrounds
     // re-runs the same normalization on every load for DBs that miss the backfill.
     private void MigrateSceneTable()
     {
@@ -1059,6 +1059,19 @@ public class LayoutStore : IDisposable
             UpsertSetting(defaultKey, bg.UseDefaultBackground ? "1" : "0");
         }
 
+        // Purge background settings left behind by deleted/duplicated rows
+        // (TASK 25 heal drops extras; their keys must not accumulate).
+        using (var purge = _connection.CreateCommand())
+        {
+            purge.Transaction = tx;
+            purge.CommandText =
+                @"DELETE FROM Settings
+                  WHERE (Key LIKE 'BackgroundUseDefault\_%' ESCAPE '\'
+                      OR Key LIKE 'BackgroundPath\_%' ESCAPE '\')
+                  AND SUBSTR(Key, INSTR(Key, '_') + 1) NOT IN (SELECT Id FROM Source);";
+            purge.ExecuteNonQuery();
+        }
+
         tx.Commit();
     }