feat(filters): filter the feed by several channels at once (OR)

Pick any number of channels; they OR together and every other filter applies
across the lot — "the last week's unwatched from these three".

`channelId`/`channelName` are replaced outright by `channelIds: string[]`, no
legacy reader (standing rule: no back-compat unless asked — a saved view written
against the old field simply loses its channel filter). The wire and the share
URL move with it: repeated `channel_ids` params, `?channel=a,b`. Backend ORs via
`Video.channel_id.in_()`; the count endpoint shares the same params object.

Two things the migration exposed:
- SavedViewsWidget fed DB blobs straight into serializers that now index an
  array — the FeedFilters type says nothing about what an older build wrote, so
  the blob is coerced where it enters. It white-screened the app before this.
- Chips carried channelName to label themselves. Without it they resolve names
  from the channel cache, so they now fall back to the id, not a shared
  "This channel" that would render N identical chips while the cache loads.

frontend/src/components/NotificationsPanel.tsx

@@ -86,8 +86,7 @@ export default function NotificationsPanel({
     setFilters({
       ...filters,
       show: meta.kind === "video-hidden" ? "hidden" : "watched",
-      channelId: meta.channelId || undefined,
-      channelName: meta.channelName || undefined,
+      channelIds: meta.channelId ? [meta.channelId] : [],
     });
     setPage("feed");
   }