Latency alerts only for real slowdowns

Slow-response alerts were firing too often, sometimes every hour on the same slow day. A latency alert now needs a meaningful slowdown: response times at least 1.5 times the monitor's normal level and at least 250 ms slower, measured over enough checks to be reliable.

Each monitor sends at most one latency alert per 24 hours, and each channel receives at most three a day across all monitors. Down, recovery, certificate and domain alerts are not affected.