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/pages/Home.jsx

@@ -9,6 +9,7 @@ const PAGE_SIZE = 25;
 const FEED_MODES = [
   { value: "ranked",        label: "For you",      hint: "Ranked by your taste" },
   { value: "chronological", label: "New",           hint: "Everything in date order" },
+  { value: "rediscover",    label: "Rediscover",    hint: "Older unwatched videos ranked by your taste" },
   { value: "random",        label: "Explore",       hint: "Random from discovery pool" },
   { value: "inbox",         label: "Inbox",         hint: "New from followed channels since last visit" },
 ];