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/models/__init__.py

@@ -8,6 +8,7 @@ from .base import Base
 from .credential import Credential
 from .download_event import DownloadEvent
 from .external_link import ExternalLink
+from .head_auto_apply_run import HeadAutoApplyRun
 from .head_training_run import HeadTrainingRun
 from .image_prediction import ImagePrediction
 from .image_provenance import ImageProvenance
@@ -67,6 +68,7 @@ __all__ = [
     "ImportSettings",
     "LibraryAuditRun",
     "MLSettings",
+    "HeadAutoApplyRun",
     "HeadTrainingRun",
     "TagAlias",
     "TagAllowlist",