feat(alerts): add container health alerts with log excerpt on notifications (#2225)

Add a new "ContainerHealth" alert type that fires when a Docker container's
health check reports unhealthy, and resolves when it recovers. This mirrors
the existing Status (up/down) alert pattern: an alert can be armed per system
and honors the "min minutes" delay before firing.

When the alert fires, the notification (email and any configured webhook,
including Discord via shoutrrr) includes a log excerpt fetched live from the
agent for up to 2 of the unhealthy containers, prioritizing lines containing
"error" or "fatal" (falling back to the log tail if none match), capped to
keep the message well under Discord's size limit.

---------

Co-authored-by: hank <hank@henrygd.me>

internal/site/src/components/alerts/alerts-sheet.tsx

@@ -411,6 +411,7 @@ export function AlertContent({
 							</div>
 						)}
 					</Suspense>
+          {checked && alertData.note && <span className="block col-span-full text-sm text-muted-foreground -mt-3">{alertData.note()}</span>}
 				</div>
 			)}
 		</div>