fix(backup): address Codacy findings

- api_v3: replace 'fonts' in ' '.join(result.restored) substring check
  with any(r.startswith("fonts") for r in result.restored) to avoid
  fragile joined-string membership testing
- api_v3: replace deprecated datetime.utcnow() and utcfromtimestamp()
  with datetime.now(timezone.utc) and fromtimestamp(..., timezone.utc);
  add timezone to import
- test: remove unused import io (backup_manager no longer uses BytesIO)
- src/backup_manager.py hardcoded /tmp sentinel was already fixed in a
  prior commit (tempfile.TemporaryDirectory)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/test_backup_manager.py

@@ -2,7 +2,6 @@
 
 from __future__ import annotations
 
-import io
 import json
 import zipfile
 from pathlib import Path