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.

backend/app/routes/channels.py

@@ -209,7 +209,7 @@ def discover_channels(
 
 
 def _channel_detail_dict(
-    channel: Channel, *, subscribed: bool, explored: bool, stored: int
+    channel: Channel, *, subscribed: bool, explored: bool, blocked: bool, stored: int
 ) -> dict:
     return {
         "id": channel.id,
@@ -226,6 +226,7 @@ def _channel_detail_dict(
         "country": channel.country,
         "subscribed": subscribed,
         "explored": explored,
+        "blocked": blocked,
         "stored_videos": stored,
         "from_explore": channel.from_explore,
         "details_synced": channel.details_synced_at is not None,
@@ -266,9 +267,14 @@ def channel_detail(
             ExploredChannel.user_id == user.id, ExploredChannel.channel_id == channel_id
         )
     ).first() is not None
+    blocked = db.execute(
+        select(BlockedChannel.id).where(
+            BlockedChannel.user_id == user.id, BlockedChannel.channel_id == channel_id
+        )
+    ).first() is not None
     stored = db.scalar(select(func.count(Video.id)).where(Video.channel_id == channel_id)) or 0
     return _channel_detail_dict(
-        channel, subscribed=subscribed, explored=explored, stored=int(stored)
+        channel, subscribed=subscribed, explored=explored, blocked=blocked, stored=int(stored)
     )