feat(heads): earned auto-apply — sweep mechanism, off by default (#114 auto-apply A)

Graduated heads can now apply their tag without a human — gated so it's safe:
- FIRING GATE: a head fires only when the master switch (head_auto_apply_enabled,
  default OFF) is on AND it has >= head_auto_apply_min_positives (default 30)
  clean labels. A precise-looking but under-supported low-N head can't spray tags.
- auto_apply_sweep (heads.py): streams every embedded image in chunks, scores
  against the eligible heads (numpy, no sklearn), applies each head's tag where
  score >= its auto_apply_threshold and the tag isn't already applied/rejected,
  with source='head_auto' (distinguishable + reversible). dry_run counts only.
- HeadAutoApplyRun (migration 0059) tracks each sweep / preview; apply_head_tags
  task (ml queue) + scheduled_apply_head_tags daily beat (no-op unless enabled)
  + recovery sweep + retention(20).
- API: POST /api/heads/auto-apply {dry_run} (202 / 409 running / 400 disabled),
  GET /api/heads/auto-apply (recent runs + per-concept report). Settings
  head_auto_apply_enabled + min_positives via /api/ml/settings.

Tests: sweep applies above threshold, dry-run writes nothing, skips under-
supported + ungraduated heads; API disabled/dry-run/conflict guards.

NEXT (slice 2): the observability the operator asked for — per-concept misfire
(auto-applied-then-removed) + under-fire tracking, time-series snapshots, and a
reporting API to tune. Slice 3: the UI (enable, preview, trends).

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

backend/app/celery_app.py

@@ -113,6 +113,10 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.ml.scheduled_train_heads",
                 "schedule": 86400.0,  # passive cadence; manual retrain stays available
             },
+            "apply-head-tags-daily": {
+                "task": "backend.app.tasks.ml.scheduled_apply_head_tags",
+                "schedule": 86400.0,  # no-op unless head_auto_apply_enabled
+            },
             "integrity-verify-weekly": {
                 "task": "backend.app.tasks.maintenance.verify_integrity",
                 "schedule": 604800.0,  # weekly
@@ -168,6 +172,10 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.maintenance.recover_stalled_head_training_runs",
                 "schedule": 300.0,
             },
+            "recover-stalled-head-auto-apply-runs": {
+                "task": "backend.app.tasks.maintenance.recover_stalled_head_auto_apply_runs",
+                "schedule": 300.0,
+            },
             "recover-stalled-import-batches": {
                 "task": "backend.app.tasks.maintenance.recover_stalled_import_batches",
                 "schedule": 300.0,