chore: drop three unused imports that fail Python Quality (#1514)

Python Quality is failing on new PRs because of three unused imports on
main (it did on #1513): `ipaddress` in `core/image_cache.py`,
`threading` in `core/notifications.py` and `email.utils.parseaddr` in
`download/orchestrator.py`. The code that used them changed in the last
round of merges and the imports stayed behind.

This is `ruff check --fix --select F401` on those three files and
nothing else. Ruff, basedpyright and tests/core plus tests/download
pass.

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