test(ml): bump auto-apply test head n_pos default 30→60 past the new floor (m139)

The stricter head_auto_apply_min_positives (30→50, migration 0081) dropped the
_head helper's default n_pos=30 below the support floor, so the "supported head"
sweep tests saw the head as ineligible (n_applied 0). Move the default to 60; the
explicit n_pos=5 under-supported test stays correct.

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

tests/test_head_auto_apply.py

@@ -35,7 +35,7 @@ def _img(db, sha: str, emb) -> ImageRecord:
     return img
 
 
-def _head(db, tag_id: int, slot: int, *, threshold=0.5, n_pos=30):
+def _head(db, tag_id: int, slot: int, *, threshold=0.5, n_pos=60):
     s = db.execute(select(MLSettings).where(MLSettings.id == 1)).scalar_one()
     w = [0.0] * 1152
     w[slot] = 1.0