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/celery_app.py

@@ -183,10 +183,6 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.maintenance.recover_stalled_library_audit_runs",
                 "schedule": 300.0,
             },
-            "recover-stalled-tag-eval-runs": {
-                "task": "backend.app.tasks.maintenance.recover_stalled_tag_eval_runs",
-                "schedule": 300.0,
-            },
             "recover-stalled-head-training-runs": {
                 "task": "backend.app.tasks.maintenance.recover_stalled_head_training_runs",
                 "schedule": 300.0,