chore: retire the tag-eval harness — it proved the heads system, job done (operator-approved)

The head-vs-centroid eval (#1130) existed to prove the 'frozen embedding +
trained head' spine; the operator accepted the tagging system and dropped the
harness. Removed per rule 22: TagEvalCard + store, /api/tag_eval blueprint,
tag_eval_run ml task, recover-stalled-tag-eval-runs sweep + beat entry,
TagEvalRun model + table (migration 0073), and its tests.

The eval's data loaders + metric helpers were NOT eval-specific — the nightly
heads trainer runs on them — so they moved verbatim to
services/ml/training_data.py (heads.py import updated; behavior unchanged).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_api_tag_eval.py

@@ -1,77 +0,0 @@
-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"