feat(playlists): unify Saved into a built-in Watch later playlist

The old per-video status='saved' becomes membership in a built-in, undeletable
'watch_later' playlist. Migration 0012 moves existing saved videos into each user's
Watch later list and demotes the states to 'new'. The feed/playlist serializers now
expose a 'saved' boolean (EXISTS in watch_later); the card bookmark toggles watch_later
via new /api/playlists/watch-later add/remove endpoints (create-on-demand) instead of
setting a status. Removed the 'saved' show-filter and status. Watch later shows a
localized name and hides rename/delete in the Playlists page and add-to-playlist popover.

frontend/src/components/Feed.tsx

@@ -15,8 +15,6 @@ function matchesView(status: string, show: string): boolean {
       return status === "hidden";
     case "watched":
       return status === "watched";
-    case "saved":
-      return status === "saved";
     case "unwatched":
     case "in_progress":
       // (in_progress is further narrowed server-side by resume position; here we only
@@ -42,6 +40,7 @@ export default function Feed({
 }) {
   const { t } = useTranslation();
   const [overrides, setOverrides] = useState<Record<string, string>>({});
+  const [savedOverrides, setSavedOverrides] = useState<Record<string, boolean>>({});
   // The open player: which video and where to start (null = resume from saved position).
   const [activeVideo, setActiveVideo] = useState<{ video: Video; startAt: number | null } | null>(
     null
@@ -64,8 +63,14 @@ export default function Feed({
   // arrives — after a refetch the server is authoritative, so a stale override
   // (e.g. a video reverted to "new" from the notification center) won't keep it
   // filtered out of the current view.
-  useEffect(() => setOverrides({}), [filters]);
-  useEffect(() => setOverrides({}), [query.dataUpdatedAt]);
+  useEffect(() => {
+    setOverrides({});
+    setSavedOverrides({});
+  }, [filters]);
+  useEffect(() => {
+    setOverrides({});
+    setSavedOverrides({});
+  }, [query.dataUpdatedAt]);
 
   const countQuery = useQuery({
     queryKey: ["feed-count", filters],
@@ -139,10 +144,24 @@ export default function Feed({
     [filters, setFilters]
   );
 
+  const onToggleSave = useCallback(
+    (id: string, saved: boolean) => {
+      setSavedOverrides((o) => ({ ...o, [id]: saved }));
+      (saved ? api.watchLaterAdd(id) : api.watchLaterRemove(id))
+        .then(() => {
+          qc.invalidateQueries({ queryKey: ["playlists"] });
+          qc.invalidateQueries({ queryKey: ["playlist"] });
+        })
+        .catch(() => setSavedOverrides((o) => ({ ...o, [id]: !saved })));
+    },
+    [qc]
+  );
+
   const loaded: Video[] = (query.data?.pages ?? []).flatMap((p) => p.items);
   loadedRef.current = loaded;
   const items: Video[] = loaded
     .map((v) => (overrides[v.id] ? { ...v, status: overrides[v.id] } : v))
+    .map((v) => (savedOverrides[v.id] !== undefined ? { ...v, saved: savedOverrides[v.id] } : v))
     .filter((v) => matchesView(v.status, filters.show));
 
   if (query.isLoading) return <div className="p-8 text-muted">{t("feed.loading")}</div>;
@@ -192,6 +211,7 @@ export default function Feed({
               video={v}
               view="grid"
               onState={onState}
+              onToggleSave={onToggleSave}
               onChannelFilter={onChannelFilter}
               onOpen={openVideo}
             />
@@ -205,6 +225,7 @@ export default function Feed({
               video={v}
               view="list"
               onState={onState}
+              onToggleSave={onToggleSave}
               onChannelFilter={onChannelFilter}
               onOpen={openVideo}
             />