Add older content exploration: channel page + home feed Rediscover mode

Channel page:
- "Explore older videos" button fetches 100 videos at a time further back
  in the channel history using yt-dlp --playlist-start/--playlist-end
- "Fetch entire history" still available for full crawl
- Backend: /channels/{id}/explore?page=N endpoint + playlist offset support
  in fetch_channel_metadata(start_video=N)

Home feed:
- New "Rediscover" mode: older unwatched videos (90+ days old) from
  followed channels, randomly sampled then re-ranked by tag affinity

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/index.js

@@ -48,6 +48,7 @@ export const followChannel = (id) => api.post(`/channels/${id}/follow`);
 export const unfollowChannel = (id) => api.delete(`/channels/${id}/follow`);
 export const indexChannel = (id) => api.post(`/channels/${id}/index`);
 export const indexChannelFull = (id) => api.post(`/channels/${id}/index-full`);
+export const exploreChannelOlder = (id, page) => api.post(`/channels/${id}/explore`, null, { params: { page } });
 export const followChannelByUrl = (data) => api.post("/channels/follow-by-url", data);
 export const setChannelAutoDownload = (id, value) => api.patch(`/channels/${id}/auto-download`, { auto_download: value });
 export const markChannelsSeen = () => api.post("/channels/mark-seen");