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/channel.json

@@ -16,5 +16,8 @@
   "tabVideos": "Videos",
   "tabAbout": "About",
   "loadMore": "Load more from YouTube",
-  "noDescription": "This channel has no description."
+  "noDescription": "This channel has no description.",
+  "block": "Block channel",
+  "unblock": "Unblock channel",
+  "blockedBadge": "Blocked"
 }