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/config.json

@@ -96,6 +96,10 @@
     "explore_grace_days": {
       "label": "Explore — grace period (days)",
       "hint": "How long an explored-but-unsubscribed channel (and its ephemeral videos) is kept before the cleanup job removes it. Subscribing keeps it permanently."
+    },
+    "search_grace_days": {
+      "label": "Search results — grace period (days)",
+      "hint": "How long an un-kept live-search result video (nobody watched / saved / subscribed to) is kept before the discovery-cleanup job removes it."
     }
   },
   "save": "Save",