fix: resolve ruff lint errors (E402, F401, F821)

- check_versions.py: restore correct file (import re was placed before
  shebang causing E402 for all following imports; missing 'client' in
  kubernetes import causing F821 at line 236)
- request_upgrades.py: remove unused 're' import and unused 'client'
  from kubernetes import (only 'config' is needed here)
- web.py: remove unused 'os' import

check_versions.py

@@ -1,4 +1,3 @@
-import re
 #!/usr/bin/env python3
 """
 k8s-version-tracker: Weekly Kubernetes image version checker and Mattermost notifier.
@@ -9,6 +8,8 @@ notification with the weekly report.
 """
 
 import os
+import re
+import json
 import logging
 import subprocess
 from dataclasses import dataclass, field
@@ -16,7 +17,7 @@ from typing import Optional
 from datetime import datetime, timezone
 
 import requests
-from kubernetes import config
+from kubernetes import client, config
 from packaging.version import Version, InvalidVersion
 
 logging.basicConfig(level=logging.INFO, format="%(asctime)s %(levelname)s %(message)s")
@@ -120,6 +121,7 @@ def parse_image_ref(image: str) -> ImageInfo:
 def _semver_tags(tags: list[str], current_tag: str) -> Optional[str]:
     """From a list of tags, return the latest stable semver tag comparable to current."""
     norm = current_tag.lstrip("v")
+    has_v_prefix = current_tag.startswith("v")
 
     # Detect suffix pattern (e.g. '-alpine', '-amd64')
     suffix_match = re.search(r"(-[a-zA-Z][a-zA-Z0-9._-]*)$", norm)