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

backend/app/tasks/ml.py

@@ -250,51 +250,6 @@ def backfill(self) -> int:
     return enqueued
 
 
-@celery.task(
-    name="backend.app.tasks.ml.tag_eval_run",
-    bind=True,
-    # The head-vs-centroid eval (#1130) loads embeddings + fits sklearn heads
-    # for several concepts — minutes, not seconds. Runs on the ml queue because
-    # only that worker has numpy/scikit-learn.
-    soft_time_limit=1800, time_limit=2100,
-)
-def tag_eval_run(self, run_id: int) -> str:
-    """Compute the eval report into the persisted TagEvalRun row so it survives
-    navigation (the admin card rehydrates from the row, not transient state)."""
-    from datetime import UTC, datetime
-
-    from ..models import TagEvalRun
-    from ..services.ml.tag_eval import run_eval
-
-    SessionLocal = _sync_session_factory()
-    with SessionLocal() as session:
-        run = session.get(TagEvalRun, run_id)
-        if run is None:
-            return "missing"
-        run.last_progress_at = datetime.now(UTC)
-        session.commit()
-        try:
-            report = run_eval(session, run.params)
-        except SoftTimeLimitExceeded:
-            run.status = "error"
-            run.error = "timed out"
-            run.finished_at = datetime.now(UTC)
-            session.commit()
-            raise
-        except Exception as exc:
-            log.exception("tag_eval_run %d failed", run_id)
-            run.status = "error"
-            run.error = str(exc)
-            run.finished_at = datetime.now(UTC)
-            session.commit()
-            return "error"
-        run.report = report
-        run.status = "ready"
-        run.finished_at = datetime.now(UTC)
-        session.commit()
-        return "ready"
-
-
 @celery.task(
     name="backend.app.tasks.ml.train_heads",
     bind=True,