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/channels.json

@@ -123,5 +123,9 @@
   },
   "confirmUnsubscribe": "Unsubscribe from \"{{name}}\" on YouTube? This changes your real YouTube account. To just remove it from your feed, hide it instead.",
   "confirmReset": "Re-fetch \"{{name}}\" from scratch? This re-runs its backfill (recent + full history) and spends some API quota.",
-  "searchPlaceholder": "Search channels…"
+  "searchPlaceholder": "Search channels…",
+  "blocked": {
+    "title": "Blocked channels",
+    "hint": "Their videos are hidden from your feed, Library, and live search."
+  }
 }