IPv6 Spamhaus check always reports "not blacklisted"
The IPv6 blocklist check in `management/status_checks.py` builds the DNSBL query name by reversing the 16-bit groups of the address instead of reversing individual nibbles.
The malformed name returns NXDOMAIN, which evaluate_spamhaus_lookup()` treats as "not blacklisted," so the IPv6 check always passes regardless of actual listing status.
DNSBLs (including Spamhaus) expect nibble-reversed format.
This fix presents it in the correct nibble-reversed format.
