fix(migration): 0045 backfill guards json_each against non-object rows

Some image_record rows store tagger_predictions as a JSON scalar/null rather
than an object; json_each throws 'cannot deconstruct a scalar' on those,
rolling back the whole migration. Filter to json_typeof = 'object' in a
MATERIALIZED CTE so the guard runs before json_each ever evaluates a scalar.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

alembic/versions/0045_image_prediction_table.py

@@ -55,17 +55,26 @@ def upgrade() -> None:
     # 0.70) right here so this is self-sufficient — it does NOT depend on the
     # #764 prune having run, and extracting only the >=floor tail keeps
     # image_prediction small (~tens of rows/image) even from the full JSON.
+    # MATERIALIZED so the json_typeof guard runs BEFORE json_each — some rows
+    # store tagger_predictions as a JSON scalar/null (not an object), and
+    # json_each throws "cannot deconstruct a scalar" on those. Filtering to
+    # objects in a materialized CTE keeps json_each from ever seeing them.
     op.execute(
         """
+        WITH objs AS MATERIALIZED (
+            SELECT id, tagger_predictions AS preds
+            FROM image_record
+            WHERE tagger_predictions IS NOT NULL
+              AND json_typeof(tagger_predictions) = 'object'
+        )
         INSERT INTO image_prediction (image_record_id, raw_name, category, score)
-        SELECT ir.id,
+        SELECT o.id,
                je.key,
                COALESCE(je.value ->> 'category', 'general'),
                (je.value ->> 'confidence')::double precision
-        FROM image_record ir,
-             json_each(ir.tagger_predictions) je
-        WHERE ir.tagger_predictions IS NOT NULL
-          AND je.value ->> 'confidence' IS NOT NULL
+        FROM objs o,
+             json_each(o.preds) je
+        WHERE je.value ->> 'confidence' IS NOT NULL
           AND (je.value ->> 'confidence')::double precision
               >= COALESCE(
                   (SELECT tagger_store_floor FROM ml_settings WHERE id = 1),