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/de/channels.json

@@ -123,5 +123,9 @@
   },
   "confirmUnsubscribe": "Kanal „{{name}}“ bei YouTube abbestellen? Dies ändert dein echtes YouTube-Konto. Um ihn nur aus deinem Feed zu entfernen, blende ihn stattdessen aus.",
   "confirmReset": "Kanal „{{name}}“ von Grund auf neu laden? Dies startet den Backfill neu (aktuell + vollständiger Verlauf) und verbraucht etwas API-Kontingent.",
-  "searchPlaceholder": "Kanäle suchen…"
+  "searchPlaceholder": "Kanäle suchen…",
+  "blocked": {
+    "title": "Blockierte Kanäle",
+    "hint": "Ihre Videos sind in deinem Feed, der Bibliothek und der Live-Suche ausgeblendet."
+  }
 }