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/scheduler.py

@@ -16,7 +16,7 @@ from app.models import SchedulerSetting
 from app.notifications import create_notification
 from app.state import is_sync_paused
 from app.sync.autotag import run_autotag_all
-from app.sync.explore import purge_unkept_explored
+from app.sync.explore import purge_ephemeral
 from app.sync.maintenance import run_maintenance
 from app.sync.playlists import sync_all_playlists
 from app.sync.runner import (
@@ -271,9 +271,9 @@ def _demo_reset_job() -> None:
 
 
 def _explore_cleanup_job() -> None:
-    # Hard-delete explored-but-never-kept channels and their untouched ephemeral videos after
-    # the grace period. 0 quota (pure DB), so no quota attribution.
-    _job("explore_cleanup", purge_unkept_explored)
+    # Reclaim un-kept ephemeral discovery content (explored channels + live-search results) after
+    # their grace periods. 0 quota (pure DB), so no quota attribution.
+    _job("explore_cleanup", purge_ephemeral)
 
 
 # job_id -> wrapper. The single source of truth for which jobs exist and how to run one,