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/locales/zh/zh.po

@@ -424,6 +424,14 @@ msgstr "衝突"
 msgid "Connection is down"
 msgstr "連線中斷"
 
+#: src/lib/alerts.ts
+msgid "Container"
+msgstr "容器"
+
+#: src/lib/alerts.ts
+msgid "Container Health"
+msgstr "容器健康狀態"
+
 #: src/components/routes/system.tsx
 msgid "Containers"
 msgstr "容器"
@@ -1749,6 +1757,10 @@ msgstr "當 15 分鐘平均負載超過閾值時觸發"
 msgid "Triggers when 5 minute load average exceeds a threshold"
 msgstr "當 5 分鐘平均負載超過閾值時觸發"
 
+#: src/lib/alerts.ts
+msgid "Triggers when a Docker container's health check reports unhealthy"
+msgstr "當 Docker 容器的健康檢查回報不健康狀態時觸發"
+
 #: src/lib/alerts.ts
 msgid "Triggers when any sensor exceeds a threshold"
 msgstr "當任何感應器超過閾值時觸發"
@@ -1795,6 +1807,10 @@ msgstr "當任何磁碟使用率超過閾值時觸發"
 msgid "Type"
 msgstr "類型"
 
+#: src/lib/alerts.ts
+msgid "Unhealthy"
+msgstr "不健康"
+
 #: src/components/systemd-table/systemd-table.tsx
 msgid "Unit file"
 msgstr "單元檔案"