fix: reduce false container-failed notifications, add error details to UI

Backend (health_monitor.py):
- Skip health checks for instances with no container_id
- Treat 'not_found' as error only when container was previously running
- Skip duplicate error notifications when already in error state
- Skip 'not_found' notifications for containers that never ran

Frontend (notification-item.tsx):
- Display notification.message (detailed error text)
- Add expandable Details section showing metadata (exit_code, previous_status, etc.)
- New CSS styles for message and metadata display

Quality gates: py_compile, tsc --noEmit, 80/80 tests pass

apps/web/src/components/features/notification/notification-item.tsx

@@ -1,3 +1,4 @@
+import { useState } from "react";
 import { Icon } from "../../icon";
 import { formatRelativeTime } from "../../../utils/time";
 import type { NotificationItem as NotificationItemType } from "../../../api/notifications";
@@ -17,6 +18,18 @@ const severityIconMap: Record<string, IconName> = {
 	success: "success",
 };
 
+function formatMetadataValue(value: unknown): string {
+	if (value === null || value === undefined) return "—";
+	if (typeof value === "string") return value;
+	if (typeof value === "number") return String(value);
+	if (typeof value === "boolean") return value ? "Yes" : "No";
+	try {
+		return JSON.stringify(value);
+	} catch {
+		return String(value);
+	}
+}
+
 export function NotificationItem({
 	notification,
 	onMarkRead,
@@ -24,6 +37,11 @@ export function NotificationItem({
 }: NotificationItemProps) {
 	const isUnread = notification.read_at === null;
 	const iconName = severityIconMap[notification.severity] ?? "info";
+	const [expanded, setExpanded] = useState(false);
+
+	const hasDetails =
+		!!notification.message ||
+			Object.keys(notification.metadata || {}).length > 0;
 
 	return (
 		<li
@@ -35,11 +53,36 @@ export function NotificationItem({
 			</div>
 			<div className="notification-item-content">
 				<div className="notification-item-title">{notification.title}</div>
+				{notification.message && (
+					<div className="notification-item-message">
+						{notification.message}
+					</div>
+				)}
 				<div className="notification-item-time">
 					{formatRelativeTime(notification.created_at)}
 				</div>
+				{expanded && notification.metadata && (
+					<dl className="notification-item-metadata">
+						{Object.entries(notification.metadata).map(([key, value]) => (
+							<div key={key} className="notification-metadata-row">
+								<dt>{key.replace(/_/g, " ")}</dt>
+								<dd>{formatMetadataValue(value)}</dd>
+							</div>
+						))}
+					</dl>
+				)}
 			</div>
 			<div className="notification-item-actions">
+				{hasDetails && (
+					<button
+						type="button"
+						className="notification-item-action"
+						onClick={() => setExpanded(!expanded)}
+						aria-label={expanded ? "Hide details" : "Show details"}
+					>
+						{expanded ? "Less" : "Details"}
+					</button>
+				)}
 				{isUnread && (
 					<button
 						type="button"