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/feed.json

@@ -48,6 +48,11 @@
     "noResults": "Keine YouTube-Videos gefunden.",
     "error": "YouTube-Suche fehlgeschlagen.",
     "loadMore": "Mehr laden (verbraucht Kontingent)",
-    "loadMoreFree": "Mehr laden"
+    "loadMoreFree": "Mehr laden",
+    "show": "Zeigen",
+    "ephemeralNote": "Diese Ergebnisse sind temporär — sie verschwinden automatisch, außer du siehst dir eines an oder speicherst es.",
+    "clearNow": "Jetzt löschen",
+    "cleared_one": "{{count}} Ergebnis gelöscht",
+    "cleared_other": "{{count}} Ergebnisse gelöscht"
   }
 }