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.

backfill_embeddings.py

@@ -13,7 +13,6 @@ Free tier: 1500 requests/day, so ~75 batches of 20.
 import asyncio
 import argparse
 import sys
-import time
 
 sys.path.insert(0, ".")
 from utils import load_config
@@ -79,7 +78,7 @@ async def backfill(batch_size: int = 20, dry_run: bool = False):
                 print(f"  ERROR: {b['id'][:12]} ({name[:30]}): {e}")
 
         if i + batch_size < total:
-            print(f"  Waiting 2s before next batch...")
+            print("  Waiting 2s before next batch...")
             await asyncio.sleep(2)
 
     print(f"\n=== Done: {success} success, {failed} failed, {total - success - failed} skipped ===")