chore: drop imports left unused by today's merges

`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.

shelfmark/core/image_cache.py

@@ -1,7 +1,6 @@
 """Disk-based image cache with LRU eviction."""
 
 import hashlib
-import ipaddress
 import json
 import threading
 import time

shelfmark/core/notifications.py

@@ -5,7 +5,6 @@ from __future__ import annotations
 import ipaddress
 import logging
 import socket
-import threading
 from concurrent.futures import ThreadPoolExecutor
 from contextlib import contextmanager, suppress
 from contextvars import ContextVar

shelfmark/download/orchestrator.py

@@ -9,7 +9,6 @@ import random
 import threading
 import time
 from concurrent.futures import Future, ThreadPoolExecutor
-from email.utils import parseaddr
 from functools import partial
 from pathlib import Path
 from threading import Event, Lock