feat(heads): nightly auto-retrain + inline Retrain button in Explore

Two cadences for keeping heads in sync with your tagging:
- PASSIVE: a nightly `scheduled_train_heads` beat (skips if a run is already
  in flight; creates+commits the run row before dispatching train_heads so the
  ml worker always finds it). Folds the day's accepts/rejects + newly-eligible
  concepts into the heads without anyone clicking.
- ACTIVE: a "Retrain heads" button in the Explore trail bar — bank the +/-
  feedback you just gave while walking content, without a trip to Settings.

Shared logic in a new useHeadTraining composable (trigger + poll + start/finish
toasts), used by the Explore button; reflects an already-running run (incl. the
nightly one) on mount.

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

backend/app/celery_app.py

@@ -109,6 +109,10 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.ml.apply_allowlist_tags",
                 "schedule": 86400.0,
             },
+            "train-heads-nightly": {
+                "task": "backend.app.tasks.ml.scheduled_train_heads",
+                "schedule": 86400.0,  # passive cadence; manual retrain stays available
+            },
             "integrity-verify-weekly": {
                 "task": "backend.app.tasks.maintenance.verify_integrity",
                 "schedule": 604800.0,  # weekly