fix(assets): reject booleans in sibling dimension validation (use type-is)

Per CodeRabbit follow-up on #13991: bool is a subclass of int in Python,
so isinstance(True, int) is True. The previous strict-int gate would
have accepted width=True (truthy + > 0) as a valid dimension.
Realistic occurrence is low (extract_image_dimensions returns proper
ints, JSON doesn't serialize bools as numbers), but the validation gate
exists for defense-in-depth so it should be actually strict.

app/assets/services/ingest.py

@@ -414,8 +414,8 @@ def _backfill_image_dimensions_from_siblings(
         width = meta.get("width")
         height = meta.get("height")
         if (
-            not isinstance(width, int)
+            type(width) is not int
-            or not isinstance(height, int)
+            or type(height) is not int
             or width <= 0
             or height <= 0
         ):