Live response-time & host graphs on the status page

The status page now shows **response-time graphs** for probed services and
**CPU / RAM / disk** graphs for the hosting box, right next to the uptime bars.

- Captured the latency our heartbeat probe already measured
- Added a stdlib host-resource sampler (no new dependencies)
- Drew the charts as inline SVG, no charting library needed

Read the [full writeup on the blog](/blog/building-live-status-graphs).