chore: dead code cleanup (pyflakes-clean)

- Remove unused imports: time/asyncio/Path/Optional/re/Counter/jieba
  across backfill_embeddings, bucket_manager, embedding_engine, import_memory
- Drop unused local var `old_primary` in reclassify_domains
- Fix 3 placeholder-less f-strings in backfill_embeddings, migrate_to_domains,
  reclassify_domains
- Delete legacy quick-check scripts test_smoke.py / test_tools.py (replaced
  by tests/ pytest suite, never collected by pytest)
- Delete backup_20260405_2124/ (stale local snapshot, no code references)

Verified: pyflakes *.py → 0 warnings; pytest tests/ → 237 passed, 7 skipped.

import_memory.py

@@ -19,10 +19,8 @@ import os
 import json
 import hashlib
 import logging
-import asyncio
 from datetime import datetime
 from pathlib import Path
-from typing import Optional
 
 from utils import count_tokens_approx, now_iso