feat(ml): tag-eval backend — head-vs-centroid learning-curve eval (persisted)

Slice 1 of milestone #114 (tagging v2). Proves the frozen-embedding + trained-
head spine on the operator's own data, reusing the SigLIP embeddings already
stored on image_record — no re-embedding, no GPU.

Per concept: train a logistic-regression HEAD (positives + negatives = explicit
rejections + sampled unlabeled) vs the old single-CENTROID baseline; report
cross-validated precision/recall/AP for both, a LEARNING CURVE (AP/F1 as tagged
positives grow 10→30→100→300), and example image ids (head-would-suggest /
head-doubts-positive) to eyeball.

Persisted so the report SURVIVES navigation (operator-flagged): the run + full
report live in a new tag_eval_run row (mirrors library_audit_run); the admin
card will rehydrate from GET on mount, not transient state.

- models.TagEvalRun + migration 0056; runs on the ml queue (only worker with
  numpy/sklearn) — numpy/sklearn lazy-imported so the API can still enqueue.
- services/ml/tag_eval (compute + start helper, one-running guard), tasks.ml
  .tag_eval_run, api/tag-eval (POST create, GET history light / detail w/ report).
- recover_stalled_tag_eval_runs sweep + retention (keep last 20) + 5-min beat
  (rule 89). scikit-learn added to requirements-ml.
- tests: param normalization + the rehydrate read-path + create/conflict.

Frontend admin card (trigger + render persisted report) follows next.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_api_tag_eval.py

@@ -0,0 +1,77 @@
+import pytest
+
+from backend.app.models import TagEvalRun
+from backend.app.services.ml.tag_eval import (
+    DEFAULT_CONCEPTS,
+    _normalize_params,
+)
+
+pytestmark = pytest.mark.integration
+
+
+def test_normalize_params_defaults_and_overrides():
+    d = _normalize_params(None)
+    assert d["concepts"] == DEFAULT_CONCEPTS
+    assert d["neg_ratio"] >= 1 and d["cv_folds"] >= 2
+    over = _normalize_params(
+        {"concepts": ["glasses", " ", "cat"], "neg_ratio": "4",
+         "cv_folds": "1", "curve_points": [30, 10, 10]}
+    )
+    assert over["concepts"] == ["glasses", "cat"]   # blanks dropped
+    assert over["neg_ratio"] == 4
+    assert over["cv_folds"] == 2                      # clamped to >=2
+    assert over["curve_points"] == [10, 30]           # deduped + sorted
+
+
+@pytest.mark.asyncio
+async def test_history_and_detail_rehydrate(client, db):
+    # A finished run with a report — the persisted row IS the survives-navigation
+    # source: history is light (no report), detail carries it.
+    run = TagEvalRun(
+        params={"concepts": ["glasses"]},
+        status="ready",
+        report={"concepts": [{"name": "glasses", "head": {"ap": 0.9}}]},
+    )
+    db.add(run)
+    await db.flush()
+    await db.commit()
+    rid = run.id
+
+    h = await client.get("/api/tag-eval?limit=10")
+    assert h.status_code == 200
+    hbody = await h.get_json()
+    row = next(r for r in hbody["runs"] if r["id"] == rid)
+    assert row["status"] == "ready"
+    assert "report" not in row                        # list stays light
+
+    d = await client.get(f"/api/tag-eval/{rid}")
+    assert d.status_code == 200
+    dbody = await d.get_json()
+    assert dbody["report"]["concepts"][0]["name"] == "glasses"
+
+
+@pytest.mark.asyncio
+async def test_create_enqueues_running(client, db, monkeypatch):
+    monkeypatch.setattr(
+        "backend.app.tasks.ml.tag_eval_run.delay", lambda *a, **k: None
+    )
+    resp = await client.post("/api/tag-eval", json={"params": {"concepts": ["cat"]}})
+    assert resp.status_code == 202
+    body = await resp.get_json()
+    assert body["status"] == "running"
+    got = await db.get(TagEvalRun, body["run_id"])
+    assert got is not None and got.status == "running"
+
+
+@pytest.mark.asyncio
+async def test_create_conflicts_when_one_running(client, db, monkeypatch):
+    monkeypatch.setattr(
+        "backend.app.tasks.ml.tag_eval_run.delay", lambda *a, **k: None
+    )
+    db.add(TagEvalRun(params={}, status="running"))
+    await db.flush()
+    await db.commit()
+    resp = await client.post("/api/tag-eval", json={"params": {}})
+    assert resp.status_code == 409
+    body = await resp.get_json()
+    assert body["error"] == "eval_already_running"