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/AddToPlaylist.tsx

@@ -165,7 +165,9 @@ export default function AddToPlaylist({
                   >
                     {pl.has_video && <Check className="w-3 h-3" />}
                   </span>
-                  <span className="truncate">{pl.name}</span>
+                  <span className="truncate">
+                    {pl.kind === "watch_later" ? t("playlists.watchLater") : pl.name}
+                  </span>
                 </button>
               ))}
             </div>