feat(search): ephemeral results UX, count selector, channel blocklist (frontend)

- Live-search view: a results-count selector (20/40/60/100) replaces manual load-more (the free
  scrape source pages until that many are gathered); an 'these results are temporary' banner with
  a 'Clear now' button that discards them 'as if never added' (api.clearSearch) and returns to
  the feed.
- Channel blocklist: a Block/Unblock toggle + 'Blocked' badge on the channel page (blocked
  channels don't auto-explore and their videos are hidden), and a 'Blocked channels' section in
  the Channel manager with one-click unblock. ChannelDetail.blocked from the backend.
- Admin: a 'Purge discovery' button on the Scheduler page (immediate un-kept search/explore
  cleanup). EN/HU/DE throughout.

frontend/src/i18n/locales/en/scheduler.json

@@ -92,5 +92,8 @@
     "shortsPendingHint": "Enriched videos still awaiting the Shorts probe.",
     "liveRefresh": "Live to refresh",
     "liveRefreshHint": "Live/upcoming videos (and just-ended streams without a duration yet) re-checked until they settle."
-  }
+  },
+  "purgeDiscovery": "Purge discovery",
+  "purgeDiscoveryHint": "Remove all un-kept discovery content now (search results nobody watched/saved, and explored-but-unsubscribed channels) — ignoring the grace period.",
+  "purgedDiscovery": "Removed {{count}} discovery items"
 }