feat(heads): production per-concept heads — train + score backend (#114 A)

The eval (#1130) proved the frozen-embedding + trained-head spine; this lands
its production form (the first of three slices that make heads the suggestion
source, replacing Camie + centroid).

- tag_head: one logistic-regression head per general/character concept with
  enough labelled positives. Weights (pgvector), honest CV-derived suggest
  threshold + earned-auto-apply point, and per-concept quality metrics.
- head_training_run: persisted batch lifecycle (mirrors tag_eval_run) so the
  admin card shows live + historical status across navigation.
- services/ml/heads.py: TRAIN (sync, ml worker, reuses tag_eval's proven data
  loaders + metric math so production heads match measured eval numbers) and
  SCORE (async, API worker — numpy via pgvector, no scikit-learn): score one
  image's embedding against all heads → the rail's suggestions, cached on
  (count, max trained_at) so a retrain invalidates without per-request loads.
- tasks.ml.train_heads (ml queue, commits per head so a kill leaves progress)
  + recover_stalled_head_training_runs sweep + retention(20) + 5-min beat
  (rule 89).
- api/heads.py: POST /api/heads/train (one run at a time, 409 guard) + GET
  /api/heads (count, graduated, last-trained, running, per-concept table,
  recent runs).
- ml_settings: head_min_positives + head_auto_apply_precision, tunable via
  /api/ml/settings.

Scoring isn't wired into the rail yet (slice C) and the admin UI is slice B —
this slice makes training + scoring exist and CI-verifiable. 'precision' column
stored as precision_cv (SQL reserved word). Migration 0058.

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

backend/app/api/ml_admin.py

@@ -17,6 +17,8 @@ _EDITABLE = (
     "video_frame_interval_seconds",
     "video_max_frames",
     "video_min_tag_frames",
+    "head_min_positives",
+    "head_auto_apply_precision",
 )
 
 
@@ -40,6 +42,8 @@ async def get_settings():
             "video_min_tag_frames": s.video_min_tag_frames,
             "tagger_model_version": s.tagger_model_version,
             "embedder_model_version": s.embedder_model_version,
+            "head_min_positives": s.head_min_positives,
+            "head_auto_apply_precision": s.head_auto_apply_precision,
         }
     )
 
@@ -100,6 +104,11 @@ def _validate(p: dict) -> str | None:
         return "video_min_tag_frames must be >= 1"
     if p["video_min_tag_frames"] > p["video_max_frames"]:
         return "video_min_tag_frames cannot exceed video_max_frames"
+    # Head training (#114).
+    if int(p["head_min_positives"]) < 1:
+        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"
     return None