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/admin.py

@@ -376,3 +376,15 @@ def reset_demo(
     same automatically (admin-tunable interval); this is the admin's on-demand button."""
     demo = get_or_create_demo_user(db)
     return {"reset": True, "playlists_seeded": reset_demo_state(db, demo)}
+
+
+@router.post("/purge-discovery")
+def purge_discovery(
+    _: User = Depends(admin_user), db: Session = Depends(get_db)
+) -> dict:
+    """Reclaim all un-kept discovery content NOW (ignoring the grace period): live-search result
+    videos nobody watched/saved/subscribed to, plus explored-but-unsubscribed channels. The
+    scheduled discovery-cleanup job does the same on its interval; this is the on-demand button."""
+    from app.sync.explore import purge_unkept_explored, purge_unkept_search
+
+    return {**purge_unkept_search(db, grace_days=0), **purge_unkept_explored(db)}