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/api/admin.py

@@ -1,13 +1,12 @@
 """FC-3k: /api/admin — destructive admin actions.
 
-Five action surfaces:
+Action surfaces:
   POST   /api/admin/artists/<slug>/cascade-delete  (Tier C)
   POST   /api/admin/images/bulk-delete             (Tier C)
   DELETE /api/admin/tags/<int:tag_id>              (Tier B)
   POST   /api/admin/tags/<int:dest_id>/merge       (Tier B)
   POST   /api/admin/tags/prune-unused              (Tier A)
   POST   /api/admin/posts/prune-bare               (Tier A)
-  POST   /api/admin/tags/purge-legacy              (Tier A)
   GET    /api/admin/tags/<int:tag_id>/usage-count  (helper)
 
 Tier-C ops take a dry_run body flag (returns projection inline,
@@ -277,19 +276,6 @@ async def posts_reconcile_duplicates():
     return await _run_dry_run_op(reconcile_duplicate_posts, source_id=source_id)
 
 
-@admin_bp.route("/tags/purge-legacy", methods=["POST"])
-async def tags_purge_legacy():
-    """Tier-A: delete legacy IR-migration tags — archive/post/artist
-    kinds (e.g. `BlenderKnight:Hannah_BJ_Loops`) PLUS general tags with
-    a legacy name prefix (`source:*`, from IR's source kind that fell
-    back to general). dry-run preview returns per-kind + per-prefix
-    counts + a sample so the UI shows exactly what'll go before the
-    operator confirms with dry_run=false."""
-    from ..services.cleanup_service import purge_legacy_tags
-
-    return await _run_dry_run_op(purge_legacy_tags)
-
-
 @admin_bp.route("/tags/reset-content", methods=["POST"])
 async def tags_reset_content():
     """Tier-A: delete ALL general + character tags (the Camie-suggestable