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/models.py

@@ -435,6 +435,25 @@ class ExploredChannel(Base, TimestampMixin):
     )
 
 
+class BlockedChannel(Base, TimestampMixin):
+    """Per-user channel blocklist. Videos from a blocked channel are excluded from this user's
+    live YouTube search (and not ingested on their behalf) and hidden from their feed / explore
+    views — a personal "never show me this channel again" that doesn't affect other users."""
+
+    __tablename__ = "blocked_channels"
+    __table_args__ = (
+        UniqueConstraint("user_id", "channel_id", name="uq_user_blocked_channel"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"), index=True
+    )
+    channel_id: Mapped[str] = mapped_column(
+        ForeignKey("channels.id", ondelete="CASCADE"), index=True
+    )
+
+
 class ApiQuotaUsage(Base):
     """Tracks YouTube Data API units spent per Pacific-time day (the quota resets at
     midnight Pacific). The whole app shares one daily budget."""