fix: ruff C416 (dict() over comprehension) + frontend test playlistIds rename

- heads.py: conf_map = dict(conf) instead of a dict comprehension (ruff C416).
- postCard.spec.js: the modal-playlist rename (postImageIds→playlistIds) missed
  this frontend test (grep was src-only); update the expected call args.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/services/ml/heads.py

@@ -197,7 +197,7 @@ def _head_fingerprints(session: Session, tag_ids: list[int]) -> dict[int, str]:
         .where(TagPositiveConfirmation.tag_id.in_(tag_ids))
         .group_by(TagPositiveConfirmation.tag_id)
     ).all()
-    conf_map = {t: c for t, c in conf}
+    conf_map = dict(conf)
     out = {}
     for t in tag_ids:
         pc, pm = pos_map.get(t, (0, None))