fix(heads): .all() before dict() in snapshot_head_metrics

dict(session.execute(...)) on a bare Result invokes the mapping protocol (a
Result has .keys() = column names) and subscripts it → "CursorResult is not
subscriptable". Materialize with .all() so dict() consumes rows as key-value
pairs. The API path already did this; the snapshot task missed it. Caught by
test_snapshot_records_timeseries_point (run 1628).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

backend/app/tasks/maintenance.py

@@ -882,18 +882,22 @@ def snapshot_head_metrics() -> int:
                 )
             )
         }
+        # .all() first: dict() of a bare Result tries the mapping protocol (a
+        # Result exposes .keys()) and subscripts it, which fails.
         applied = dict(
             session.execute(
                 select(image_tag.c.tag_id, func.count())
                 .where(image_tag.c.source == "head_auto")
                 .group_by(image_tag.c.tag_id)
-            )
+            ).all()
         )
         tag_ids = set(heads) | set(metrics)
         if not tag_ids:
             return 0
         names = dict(
-            session.execute(select(Tag.id, Tag.name).where(Tag.id.in_(tag_ids)))
+            session.execute(
+                select(Tag.id, Tag.name).where(Tag.id.in_(tag_ids))
+            ).all()
         )
         for tid in tag_ids:
             h = heads.get(tid)