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/migrations/0_collections_snapshot_0_19_0_dev_2.go

@@ -79,7 +79,8 @@ func init() {
 					"LoadAvg1",
 					"LoadAvg5",
 					"LoadAvg15",
-					"Battery"
+					"Battery",
+					"ContainerHealth"
 				]
 			},
 			{
@@ -115,6 +116,17 @@ func init() {
 				"system": false,
 				"type": "bool"
 			},
+			{
+				"hidden": true,
+				"id": "date1302749137",
+				"max": "",
+				"min": "",
+				"name": "pending_since",
+				"presentable": false,
+				"required": false,
+				"system": false,
+				"type": "date"
+			},
 			{
 				"hidden": false,
 				"id": "autodate2990389176",