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/ml_settings.py

@@ -2,7 +2,15 @@
 
 from datetime import datetime
 
-from sqlalchemy import CheckConstraint, DateTime, Float, Integer, String, func
+from sqlalchemy import (
+    Boolean,
+    CheckConstraint,
+    DateTime,
+    Float,
+    Integer,
+    String,
+    func,
+)
 from sqlalchemy.orm import Mapped, mapped_column
 
 from .base import Base
@@ -66,6 +74,17 @@ class MLSettings(Base):
     head_auto_apply_precision: Mapped[float] = mapped_column(
         Float, nullable=False, default=0.97
     )
+    # Earned auto-apply (#114). A graduated head fires (tags images without a
+    # human) ONLY when this master switch is on AND the head has at least
+    # head_auto_apply_min_positives clean labels — so a precise-looking but
+    # under-supported low-N head can't spray tags across the library. Off by
+    # default; the operator enables after previewing. Operator-tunable.
+    head_auto_apply_enabled: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=False
+    )
+    head_auto_apply_min_positives: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=30
+    )
     tagger_model_version: Mapped[str] = mapped_column(
         String(128), nullable=False, default="camie-tagger-v2"
     )