feat(notifications): notify on 'watched' with an Unwatch action

Mirror the hide flow for watched: marking a video watched (card, modal toggle,
or auto-watch) raises a toast and a bell-history entry with an Unwatch action —
no Find-in-feed, just revert. NotifMeta becomes a discriminated union
(video-hidden | video-watched); unhide/unwatch share one revert-to-new helper.

frontend/src/lib/notifications.ts

@@ -12,13 +12,19 @@ export interface NotifAction {
 
 // Structured payload that lets the Notification Center offer in-app actions even
 // after a reload (when live `action` callbacks are gone).
-export type NotifMeta = {
+export type VideoHiddenMeta = {
   kind: "video-hidden";
   videoId: string;
   title: string;
   channelId: string;
   channelName: string;
 };
+export type VideoWatchedMeta = {
+  kind: "video-watched";
+  videoId: string;
+  title: string;
+};
+export type NotifMeta = VideoHiddenMeta | VideoWatchedMeta;
 
 export interface Notification {
   id: number;