chore: retire ml-backfill-daily beat + the spent purge-legacy action (operator-approved)

- ml-backfill-daily: the CPU tag_and_embed backfill raced the GPU agent's
  daily embed backfill for the same NULL-embedding images at ~100x the cost
  (B1 audit verdict, milestone #124). The backfill TASK stays — the manual
  /api/ml/backfill button remains the deliberate CPU fallback pending B3.
- purge-legacy: one-time IR-migration cleanup, dry-run verified 0 targets on
  the live library before removal (A2 audit, milestone #123). Fully retired
  per rule 22: tile, store action, route, service fn, tests.

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

backend/app/services/cleanup_service.py

@@ -718,71 +718,6 @@ def reconcile_duplicate_posts(
     return {"groups": len(groups), "merged": losers_total, "sample": sample}
 
 
-# Legacy tags FC no longer uses, in two shapes:
-#   (1) kinds the tag input never produces — archive/post/artist.
-#       provenance (post grouping) + archive membership are their own
-#       systems now, and artists are first-class Artist/Source rows.
-#       meta/rating were already hard-deleted by alembic 0023.
-#   (2) name prefixes from IR kinds FC never adopted — `source:*`.
-#       ImageRepo had a `source` kind; FC's enum doesn't, so ir_ingest
-#       fell those back to `general` (kind=general, name="source:patreon"
-#       etc.). They can't be caught by kind, so we match the name prefix.
-PURGEABLE_TAG_KINDS = ("archive", "post", "artist")
-LEGACY_NAME_PREFIXES = ("source:",)
-
-
-def _legacy_tag_predicate():
-    name_clauses = [Tag.name.like(f"{p}%") for p in LEGACY_NAME_PREFIXES]
-    return or_(Tag.kind.in_(PURGEABLE_TAG_KINDS), *name_clauses)
-
-
-def purge_legacy_tags(session: Session, *, dry_run: bool = False) -> dict:
-    """Count (dry_run) or delete legacy IR-migration tags: archive/post/
-    artist-kind tags PLUS general tags whose name matches a legacy
-    prefix (source:*).
-
-    CASCADE on image_tag / tag_alias / tag_suggestion_rejection / series_page
-    clears the related rows on the parent DELETE.
-
-    Returns:
-      {"by_kind": {kind: count, ...},        # kind-matched rows
-       "by_prefix": {"source:*": count},     # name-prefix-matched rows
-       "count": total, "sample_names": [first 50],
-       and on live runs "deleted": total}
-    """
-    predicate = _legacy_tag_predicate()
-    rows = session.execute(
-        select(Tag.id, Tag.name, Tag.kind).where(predicate)
-    ).all()
-    by_kind: dict[str, int] = {}
-    by_prefix: dict[str, int] = {}
-    for _id, name, kind in rows:
-        # Classify by name-prefix first so a source:* row counts once,
-        # under the prefix bucket, regardless of its (general) kind.
-        matched_prefix = next(
-            (p for p in LEGACY_NAME_PREFIXES if name.startswith(p)), None,
-        )
-        if matched_prefix is not None:
-            label = f"{matched_prefix}*"
-            by_prefix[label] = by_prefix.get(label, 0) + 1
-        else:
-            key = kind.value if hasattr(kind, "value") else str(kind)
-            by_kind[key] = by_kind.get(key, 0) + 1
-    sample = [name for _id, name, _kind in rows[:50]]
-    total = len(rows)
-    result = {
-        "by_kind": by_kind, "by_prefix": by_prefix,
-        "count": total, "sample_names": sample,
-    }
-    if dry_run:
-        return result
-    if total:
-        session.execute(Tag.__table__.delete().where(predicate))
-        session.commit()
-    result["deleted"] = total
-    return result
-
-
 # The CONTENT vocabulary. "Reset content tagging" wipes these so the operator
 # can re-tag from scratch. fandom + series (and series_page ordering) are
 # deliberately NOT here — they're kept.