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/config.py

@@ -124,6 +124,9 @@ class Settings(BaseSettings):
     # days after it was last explored — the grace clock is ExploredChannel.created_at.
     explore_cleanup_minutes: int = 1440
     explore_grace_days: int = 7
+    # How long an un-kept live-search result (a via_search video nobody watched / saved /
+    # subscribed to) is kept before the same cleanup job removes it — "as if never added".
+    search_grace_days: int = 7
     # live_status values hidden from the feed by default. Completed-stream VODs
     # ("was_live") are real watchable content and stay visible.
     feed_default_hidden_live: str = "live,upcoming"