Paginate group fetching and reduce batch size to 500

[Copilot]

fetch_groups() and fetch_groups_by_tag() had a hard limit=1000 with no pagination—any groups beyond that were silently dropped.

• Added offset-based pagination to both fetch_groups() and fetch_groups_by_tag() so all matching groups are retrieved regardless of count
• Reduced batch size from 1000 to 500 per API request

$limit = 500;
$offset = 0;
$all_data = [];

do {
    $response = wp_remote_get(".../groups?limit=$limit&offset=$offset&fields=uid,name,tags&filter=$qry", ...);
    $data = json_decode($response['body'])->data;
    $all_data = array_merge($all_data, $data);
    $offset += $limit;
} while (count($data) === $limit);