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/channels.py

@@ -12,6 +12,7 @@ from app import quota, sysconfig
 from app.auth import current_user, has_write_scope, require_human
 from app.db import get_db
 from app.models import (
+    BlockedChannel,
     Channel,
     ChannelTag,
     ExploredChannel,
@@ -287,6 +288,12 @@ def explore_channel(
     channel = db.get(Channel, channel_id)
     if channel is None:
         raise HTTPException(status_code=404, detail="Unknown channel")
+    if db.execute(
+        select(BlockedChannel.id).where(
+            BlockedChannel.user_id == user.id, BlockedChannel.channel_id == channel_id
+        )
+    ).first() is not None:
+        raise HTTPException(status_code=403, detail="You've blocked this channel.")
     if quota.remaining_today(db) <= sysconfig.get_int(db, "backfill_quota_reserve"):
         raise HTTPException(
             status_code=429,
@@ -504,6 +511,68 @@ def unsubscribe(
     return {"unsubscribed": channel_id}
 
 
+@router.get("/blocked/list")
+def list_blocked(
+    user: User = Depends(current_user), db: Session = Depends(get_db)
+) -> list[dict]:
+    """The user's blocked channels (videos from these are excluded from their search/feed/explore)."""
+    rows = db.execute(
+        select(Channel)
+        .join(BlockedChannel, BlockedChannel.channel_id == Channel.id)
+        .where(BlockedChannel.user_id == user.id)
+        .order_by(func.lower(Channel.title))
+    ).scalars().all()
+    return [
+        {"id": c.id, "title": c.title, "handle": c.handle, "thumbnail_url": c.thumbnail_url}
+        for c in rows
+    ]
+
+
+@router.post("/{channel_id}/block")
+def block_channel(
+    channel_id: str,
+    user: User = Depends(current_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    """Block a channel for this user: its videos won't appear in their live search (and won't be
+    ingested for them) and are hidden from their feed / explore. Idempotent. The now-hidden
+    un-kept search/explore videos are reclaimed by the discovery-cleanup job in due course."""
+    if db.get(Channel, channel_id) is None:
+        raise HTTPException(status_code=404, detail="Unknown channel")
+    exists = db.execute(
+        select(BlockedChannel.id).where(
+            BlockedChannel.user_id == user.id, BlockedChannel.channel_id == channel_id
+        )
+    ).first()
+    if exists is None:
+        db.add(BlockedChannel(user_id=user.id, channel_id=channel_id))
+        # Stop any active exploration of it so it can be cleaned up.
+        db.execute(
+            ExploredChannel.__table__.delete().where(
+                (ExploredChannel.user_id == user.id)
+                & (ExploredChannel.channel_id == channel_id)
+            )
+        )
+        db.commit()
+    return {"blocked": channel_id}
+
+
+@router.delete("/{channel_id}/block")
+def unblock_channel(
+    channel_id: str,
+    user: User = Depends(current_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    db.execute(
+        BlockedChannel.__table__.delete().where(
+            (BlockedChannel.user_id == user.id)
+            & (BlockedChannel.channel_id == channel_id)
+        )
+    )
+    db.commit()
+    return {"unblocked": channel_id}
+
+
 @router.post("/{channel_id}/tags")
 def attach_tag(
     channel_id: str,