feat(alerts): a rule can fire when a reading rises, "rose by N" (#6724)

A rule compared a reading with a fixed number, so Synology Disk 1's bad
sectors going from 3,160 to 3,392 could say nothing: a count that only grows
is soon past any line. "rose by" compares each reading with the one before it,
kept on the alert's state and only for such a rule. A fall (a replaced disk)
doesn't fire. On the rule form, through MCP, and worded in emails, the alert
sentence, history and the timeline.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/core/alerting.md

@@ -73,6 +73,7 @@ Alert rules are created in the UI at `/alerts/`. Each rule requires:
 - **Metric name** — the metric key (e.g. `response_time_ms`, `up`, `error_rate_5xx_pct`)
 - **Operator** — `>`, `<`, `>=`, `<=`, `==`, `!=`
 - **Threshold** — numeric value
+- **rose by** — instead of a line to cross, fires when a reading is at least the threshold above the one before it, for a count that only grows (a disk's bad sectors). A fall doesn't fire. A reading that hasn't risen counts as OK, so the checks to clear set how long the alert stays up, and rises inside that are one alert
 - **Consecutive failures required** — how many consecutive breaches before FIRING (default 1)
 
 The form offers only what has reported, read from `plugin_metric_latest`.