Adds DNS_FORWARD_BUDGET (default 50) and DNS_FORWARD_WINDOW (default 1.0s)
env vars. _can_forward() maintains a rolling deque of upstream call
timestamps; queries that exceed the budget within the window are answered
with the sinkhole (127.x) instead of being forwarded, making the honeypot
ineligible as a sustained amp vector even when real_recursive is enabled.
Rate limit is global (not per-source) so IP-spoofed amplification floods
hit the ceiling regardless of how many source addresses are rotated.