feat: filter notifications to warnings/errors/ready only and add clear-all button

Notification filtering:
- lifecycle_hooks.py: only instance.error and instance.health_changed
  with status=running generate notifications. All other lifecycle events
  (created, started, stopped, restarted, deleted) are filtered out.
- health_monitor.py: only error and unhealthy states generate notifications.
  Running/recovered state no longer creates info notifications.
- _derive_title now maps instance.health_changed to "Container ready".

Clear-all button:
- Added dismiss_all() to NotificationService
- Added DELETE /notifications endpoint for bulk dismiss
- Frontend: clearAllNotifications API, clearAll in notification context,
  "Clear all" button in notification drawer alongside "Mark all as read"
- Added CSS for .notification-clear-all with danger hover state
- Updated notification-center tests

Quality gates: pytest (21 passed), vitest (11 passed)

apps/web/src/state/notifications.tsx

@@ -11,6 +11,7 @@ import {
 	markNotificationRead,
 	markAllNotificationsRead,
 	dismissNotification,
+	clearAllNotifications,
 } from "../api/notifications";
 import type { NotificationItem } from "../api/notifications";
 
@@ -21,6 +22,7 @@ export interface NotificationContextValue {
 	error: Error | null;
 	markRead: (id: string) => Promise<void>;
 	markAllRead: () => Promise<void>;
+	clearAll: () => Promise<void>;
 	dismiss: (id: string) => Promise<void>;
 	refreshList: () => Promise<void>;
 	isDropdownOpen: boolean;
@@ -265,6 +267,24 @@ export function NotificationProvider({
 		await fetchList();
 	}, [fetchList]);
 
+	const clearAll = useCallback(async () => {
+		const { notifications: currentNotifications } = stateRef.current;
+		const unreadInList = currentNotifications.filter(
+			(n) => n.read_at === null,
+		).length;
+
+		setNotifications([]);
+		setUnreadCount((c) => Math.max(0, c - unreadInList));
+		setError(null);
+
+		try {
+			await clearAllNotifications();
+		} catch (err) {
+			setNotifications(currentNotifications);
+			setError(err as Error);
+		}
+	}, []);
+
 	const value: NotificationContextValue = {
 		notifications,
 		unreadCount,
@@ -272,6 +292,7 @@ export function NotificationProvider({
 		error,
 		markRead,
 		markAllRead,
+		clearAll,
 		dismiss,
 		refreshList,
 		isDropdownOpen,