fix: correct repo URL fallback and HTTP error reporting in update service

- Replace hardcoded legacy Gitea URL in settings.py fallback
- Skip legacy Gitea URL even if saved in DB (treat as unset)
- Track last_response in branch loop so errors report the real HTTP
  status code instead of always showing "HTTP unknown"
- Unify 401/403/404 into one "private or not accessible" message

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

backend/app/api/settings.py

@@ -706,7 +706,7 @@ async def get_repository_url(
     setting = db.query(PanelSettings).filter(PanelSettings.key == "git_repository_url").first()
     if setting:
         return {"repository_url": setting.value}
-    return {"repository_url": "https://git.tzim.uz/dilshod/pve_manager"}
+    return {"repository_url": "https://github.com/markmorado/pvemanager"}
 
 
 @router.put("/api/updates/repository")