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/maintenance.py

@@ -21,7 +21,6 @@ from ..models import (
     ImportTask,
     LibraryAuditRun,
     Source,
-    TagEvalRun,
     TaskRun,
 )
 from ..utils.phash import compute_phash
@@ -96,9 +95,6 @@ BACKUP_DB_STALL_THRESHOLD_MINUTES = 40
 # Library audit: scan_library_for_rule has time_limit=7500s (2h5m).
 # 2h15m gives a 10-min buffer.
 LIBRARY_AUDIT_STALL_THRESHOLD_MINUTES = 135
-# tag-eval (#1130) has a 30-min soft limit; flag a run with no progress past 40.
-TAG_EVAL_STALL_THRESHOLD_MINUTES = 40
-TAG_EVAL_KEEP_RUNS = 20
 # head training (#114) has a 60-min soft limit; flag no-progress past 75.
 HEAD_TRAINING_STALL_THRESHOLD_MINUTES = 75
 HEAD_TRAINING_KEEP_RUNS = 20
@@ -743,46 +739,6 @@ def recover_stalled_library_audit_runs() -> int:
         return recovered
 
 
-@celery.task(name="backend.app.tasks.maintenance.recover_stalled_tag_eval_runs")
-def recover_stalled_tag_eval_runs() -> int:
-    """Flip TagEvalRun rows stuck in 'running' past the stall threshold to
-    'error', and prune old runs to the last TAG_EVAL_KEEP_RUNS (retention,
-    rule 89). Runs every 5 min on the maintenance lane; no-op when idle."""
-    SessionLocal = _sync_session_factory()
-    now = datetime.now(UTC)
-    cutoff = now - timedelta(minutes=TAG_EVAL_STALL_THRESHOLD_MINUTES)
-    with SessionLocal() as session:
-        result = session.execute(
-            update(TagEvalRun)
-            .where(TagEvalRun.status == "running")
-            .where(
-                func.coalesce(TagEvalRun.last_progress_at, TagEvalRun.started_at)
-                < cutoff
-            )
-            .values(
-                status="error", finished_at=now,
-                error=(
-                    f"stranded by recovery sweep (no progress for "
-                    f"{TAG_EVAL_STALL_THRESHOLD_MINUTES} min)"
-                ),
-            )
-        )
-        # Retention: keep only the most recent N runs.
-        keep = session.execute(
-            select(TagEvalRun.id).order_by(TagEvalRun.id.desc())
-            .limit(TAG_EVAL_KEEP_RUNS)
-        ).scalars().all()
-        if keep:
-            session.execute(
-                delete(TagEvalRun).where(TagEvalRun.id.not_in(keep))
-            )
-        session.commit()
-        recovered = result.rowcount or 0
-        if recovered:
-            log.info("recover_stalled_tag_eval_runs: recovered %d rows", recovered)
-        return recovered
-
-
 @celery.task(name="backend.app.tasks.maintenance.recover_stalled_head_training_runs")
 def recover_stalled_head_training_runs() -> int:
     """Flip HeadTrainingRun rows stuck in 'running' past the stall threshold to