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/components/notification-center.tsx

@@ -15,6 +15,7 @@ export function NotificationCenter({
 		unreadCount,
 		markRead,
 		markAllRead,
+		clearAll,
 		dismiss,
 		refreshList,
 		isDropdownOpen,
@@ -115,6 +116,15 @@ export function NotificationCenter({
 							>
 								Mark all as read
 							</button>
+							<button
+								type="button"
+								className="notification-clear-all"
+								onClick={() => {
+									void clearAll();
+								}}
+							>
+								Clear all
+							</button>
 						</div>
 					)}
 				</div>