`ruff check` fails on main with three F401 unused imports. Each lost its
last use when code moved into a shared helper:
- image_cache: `ipaddress`, after the cover-cache address check moved to
network.hostname_resolves_publicly (f837506)
- orchestrator: `email.utils.parseaddr`, after the recipient check moved
to request_helpers.is_plain_email_address (cb72ce8)
- notifications: `threading`, which #1510 removed but its merge of main
brought back
No behavior change.