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/feed.json

@@ -48,6 +48,11 @@
     "noResults": "No YouTube videos found.",
     "error": "YouTube search failed.",
     "loadMore": "Load more (uses quota)",
-    "loadMoreFree": "Load more"
+    "loadMoreFree": "Load more",
+    "show": "Show",
+    "ephemeralNote": "These results are temporary — they clear automatically unless you watch or save one.",
+    "clearNow": "Clear now",
+    "cleared_one": "Cleared {{count}} result",
+    "cleared_other": "Cleared {{count}} results"
   }
 }