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/components/Feed.tsx

@@ -87,6 +87,13 @@ export default function Feed({
           channelName: v?.channel_title ?? "",
         },
       });
+    } else if (status === "watched") {
+      const v = (query.data?.pages ?? []).flatMap((p) => p.items).find((x) => x.id === id);
+      notify({
+        message: v?.title ? `Marked watched “${v.title}”` : "Marked watched",
+        action: { label: "Unwatch", onClick: () => onState(id, "new") },
+        meta: { kind: "video-watched", videoId: id, title: v?.title ?? "this video" },
+      });
     }
   }
 

frontend/src/components/NotificationCenter.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useRef, useState, useSyncExternalStore } from "react";
 import { useQueryClient } from "@tanstack/react-query";
-import { Bell, Eye, Search, Trash2 } from "lucide-react";
+import { Bell, Eye, RotateCcw, Search, Trash2 } from "lucide-react";
 import { api, type FeedFilters } from "../lib/api";
 import {
   clearAll,
@@ -10,7 +10,8 @@ import {
   markAllRead,
   notify,
   subscribe,
-  type NotifMeta,
+  type VideoHiddenMeta,
+  type VideoWatchedMeta,
 } from "../lib/notifications";
 import { LEVEL_STYLE } from "./Toaster";
 
@@ -53,7 +54,7 @@ export default function NotificationCenter({
     if (open) markAllRead();
   }, [open, notifications.length]);
 
-  function locateHidden(meta: NotifMeta) {
+  function locateHidden(meta: VideoHiddenMeta) {
     setFilters({
       ...filters,
       show: "hidden",
@@ -63,13 +64,13 @@ export default function NotificationCenter({
     setOpen(false);
   }
 
-  function unhide(meta: NotifMeta) {
+  function revertState(meta: VideoHiddenMeta | VideoWatchedMeta, verb: string) {
     api
       .setState(meta.videoId, "new")
       .then(() => {
         qc.invalidateQueries({ queryKey: ["feed"] });
         qc.invalidateQueries({ queryKey: ["feed-count"] });
-        notify({ level: "success", message: `Unhidden “${meta.title}”` });
+        notify({ level: "success", message: `${verb} “${meta.title}”` });
       })
       .catch(() => {});
   }
@@ -113,6 +114,7 @@ export default function NotificationCenter({
                 const { icon: Icon, color } = LEVEL_STYLE[n.level];
                 const needsAction = n.requiresInteraction && !n.dismissed;
                 const hidden = n.meta?.kind === "video-hidden" ? n.meta : null;
+                const watched = n.meta?.kind === "video-watched" ? n.meta : null;
                 return (
                   <div
                     key={n.id}
@@ -143,13 +145,23 @@ export default function NotificationCenter({
                             Find in feed
                           </button>
                           <button
-                            onClick={() => unhide(hidden)}
+                            onClick={() => revertState(hidden, "Unhidden")}
                             className="flex items-center gap-1 text-accent text-sm font-semibold hover:underline"
                           >
                             <Eye className="w-3.5 h-3.5" />
                             Unhide
                           </button>
                         </div>
+                      ) : watched ? (
+                        <div className="flex items-center gap-3 mt-1">
+                          <button
+                            onClick={() => revertState(watched, "Unwatched")}
+                            className="flex items-center gap-1 text-accent text-sm font-semibold hover:underline"
+                          >
+                            <RotateCcw className="w-3.5 h-3.5" />
+                            Unwatch
+                          </button>
+                        </div>
                       ) : (
                         n.action &&
                         !n.dismissed && (

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;