feat(search): ephemeral results, count selector, blocklist, new-first ordering (backend)

- Live-search results stay ephemeral: the discovery-cleanup job now also reclaims un-kept
  via_search videos (no watch state / not playlisted / channel not subscribed) after a grace
  period (search_grace_days), and POST /api/search/clear discards a given result set 'as if
  never added' (drops the user's search-finds + deletes the now-orphaned, un-kept videos).
  Admin POST /api/admin/purge-discovery runs it on demand (grace 0).
- Count selector: GET /api/search/youtube gains a 'limit' — the free scrape source pages
  through continuations until that many results are gathered (no manual load-more); the API
  source stays one ≤50 page (cost).
- Per-user channel blocklist (migration 0034 blocked_channels): block/unblock/list endpoints;
  blocked channels' videos are dropped from live search before ingest and hidden from the feed
  / Library / explore / channel page; explore refuses a blocked channel.
- New-first ordering: results you already have (subscribed channel, watched/in-progress/saved/
  playlisted) sink below genuinely-new discoveries, preserving relevance within each group.

backend/app/routes/feed.py

@@ -14,6 +14,7 @@ from app import quota
 from app.auth import current_user
 from app.db import get_db
 from app.models import (
+    BlockedChannel,
     Channel,
     ChannelTag,
     ExploredChannel,
@@ -198,6 +199,14 @@ def _filtered_query(
         )
     )
 
+    # Per-user channel blocklist: never show this user videos from a channel they blocked
+    # (applies to feed, Library, explore, and the channel page alike).
+    query = query.where(
+        Video.channel_id.notin_(
+            select(BlockedChannel.channel_id).where(BlockedChannel.user_id == user.id)
+        )
+    )
+
     # Source filter. In the shared Library ("all" scope) it uses the GLOBAL via_search flag
     # (search-discovered by anyone). In "my" scope it's the user's OWN provenance:
     # "organic" = your subscriptions, "search" = videos you found via your own search,