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/api/ml_admin.py

@@ -19,6 +19,8 @@ _EDITABLE = (
     "video_min_tag_frames",
     "head_min_positives",
     "head_auto_apply_precision",
+    "head_auto_apply_enabled",
+    "head_auto_apply_min_positives",
 )
 
 
@@ -44,6 +46,8 @@ async def get_settings():
             "embedder_model_version": s.embedder_model_version,
             "head_min_positives": s.head_min_positives,
             "head_auto_apply_precision": s.head_auto_apply_precision,
+            "head_auto_apply_enabled": s.head_auto_apply_enabled,
+            "head_auto_apply_min_positives": s.head_auto_apply_min_positives,
         }
     )
 
@@ -109,6 +113,8 @@ def _validate(p: dict) -> str | None:
         return "head_min_positives must be >= 1"
     if not (0.5 <= float(p["head_auto_apply_precision"]) <= 0.999):
         return "head_auto_apply_precision must be between 0.5 and 0.999"
+    if int(p["head_auto_apply_min_positives"]) < 1:
+        return "head_auto_apply_min_positives must be >= 1"
     return None