Let latency affect color (slow-but-up goes yellow)

[bdpedigo]

status_color in src/echo/views.py takes only latest_ok and window_rate, so latency never affects color. A service that has slowed from 200 ms to 25 s but still returns 2xx shows green, even though "up but slow" is a common degradation. The latency is already recorded and plotted, it just does not feed the color.

Approach

• Add an optional per-probe latency budget that turns a probe yellow when exceeded. Red stays for hard failures.
• Alternative that needs no tuning: turn yellow when the latest latency is some multiple of the probe's recent median. The history is already available.
• Keep the color logic in echo.views.
• Might want to document this as a different color to make the difference in meaning distinct.

Acceptance criteria

• A slow but successful probe can render yellow via a budget or a relative rule.
• Color logic stays in echo.views.