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/i18n/locales/en/playlists.json

@@ -1,5 +1,6 @@
 {
   "title": "Playlists",
+  "watchLater": "Watch later",
   "noneYet": "No playlists yet",
   "newPlaylist": "New playlist…",
   "itemCount_one": "{{count}} video",