feat(ml): soft auto-apply — retract auto-tags now below threshold (milestone 139)

Daily scheduled_retract_auto_tags re-scores standing auto-applied tags and drops
the ones the model no longer supports:
- retract_auto_applied_heads: per graduated head, re-score its source='head_auto'
  images (bounded — only the images already carrying the auto-tag, not the whole
  library) and remove ones now < auto_apply_threshold.
- retract_auto_applied_ccip: per source='ccip_auto' character tag, max-cosine the
  image's figure vectors vs that character's prototypes; remove ones now below the
  ccip auto-apply threshold.
Both SKIP operator-confirmed tags (TagPositiveConfirmation) and are SILENT — a low
score isn't proof the tag was wrong, so no hard negative is recorded (that's
reserved for an operator removal). No-op unless the relevant auto-apply switch is
on. New daily beat. sklearn-free tests for both paths + the disabled no-op.

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

backend/app/celery_app.py

@@ -147,6 +147,11 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.ml.scheduled_ccip_auto_apply",
                 "schedule": 86400.0,  # no-op unless ccip_auto_apply_enabled
             },
+            "retract-auto-tags-daily": {
+                "task": "backend.app.tasks.ml.scheduled_retract_auto_tags",
+                "schedule": 86400.0,  # soft auto-apply: drop auto-tags now below
+                # their threshold (m139); no-op unless the auto-apply switch is on
+            },
             "snapshot-head-metrics-daily": {
                 "task": "backend.app.tasks.maintenance.snapshot_head_metrics",
                 "schedule": 86400.0,