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

alembic/versions/0058_tag_head.py

@@ -0,0 +1,95 @@
+"""tag_head + head_training_run: production heads that learn from tags (#114)
+
+The eval (#1130) proved the frozen-embedding + trained-head spine; this lands its
+production form. tag_head stores one logistic-regression head per concept (the
+new suggestion source, replacing Camie + centroid); head_training_run tracks the
+batch that (re)trains them. Adds two head-training tunables to ml_settings.
+
+Revision ID: 0058
+Revises: 0057
+Create Date: 2026-06-28
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+from pgvector.sqlalchemy import Vector
+from sqlalchemy.dialects.postgresql import JSONB
+
+revision: str = "0058"
+down_revision: Union[str, None] = "0057"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+_HEAD_DIM = 1152
+
+
+def upgrade() -> None:
+    op.create_table(
+        "tag_head",
+        sa.Column(
+            "tag_id", sa.Integer(),
+            sa.ForeignKey("tag.id", ondelete="CASCADE"), primary_key=True,
+        ),
+        sa.Column("embedding_version", sa.String(length=128), nullable=False),
+        sa.Column("weights", Vector(_HEAD_DIM), nullable=False),
+        sa.Column("bias", sa.Float(), nullable=False),
+        sa.Column("suggest_threshold", sa.Float(), nullable=False),
+        sa.Column("auto_apply_threshold", sa.Float(), nullable=True),
+        sa.Column("n_pos", sa.Integer(), nullable=False),
+        sa.Column("n_neg", sa.Integer(), nullable=False),
+        sa.Column("ap", sa.Float(), nullable=False),
+        sa.Column("precision_cv", sa.Float(), nullable=False),
+        sa.Column("recall", sa.Float(), nullable=False),
+        sa.Column(
+            "trained_at", sa.DateTime(timezone=True), nullable=False,
+            server_default=sa.func.now(),
+        ),
+        sa.Column("metrics", JSONB(), nullable=True),
+    )
+
+    op.create_table(
+        "head_training_run",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column("params", JSONB(), nullable=False),
+        sa.Column(
+            "status", sa.String(length=16), nullable=False,
+            server_default="running",
+        ),
+        sa.Column(
+            "started_at", sa.DateTime(timezone=True), nullable=False,
+            server_default=sa.func.now(),
+        ),
+        sa.Column("finished_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("n_trained", sa.Integer(), nullable=True),
+        sa.Column("n_skipped", sa.Integer(), nullable=True),
+        sa.Column("error", sa.Text(), nullable=True),
+        sa.Column("last_progress_at", sa.DateTime(timezone=True), nullable=True),
+    )
+    op.create_index(
+        "ix_head_training_run_status", "head_training_run", ["status"],
+    )
+
+    # Head-training tunables on the ml_settings singleton.
+    op.add_column(
+        "ml_settings",
+        sa.Column(
+            "head_min_positives", sa.Integer(), nullable=False,
+            server_default="8",
+        ),
+    )
+    op.add_column(
+        "ml_settings",
+        sa.Column(
+            "head_auto_apply_precision", sa.Float(), nullable=False,
+            server_default="0.97",
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("ml_settings", "head_auto_apply_precision")
+    op.drop_column("ml_settings", "head_min_positives")
+    op.drop_index("ix_head_training_run_status", table_name="head_training_run")
+    op.drop_table("head_training_run")
+    op.drop_table("tag_head")