refactor(catalog): the HTTP layer reads through queries and writes through the service

Channels, search and sync do no database work in their routes any more: reads go through
catalog/queries, writes through catalog/service, and a live search's ingest is catalog/search.
What a catalog route needs from Reader (a playlist's channels, a user's tag links, the feed
projection of results) it asks reader/queries and reader/service. The scheduler and the runs page
read channel titles and the work queue through catalog/queries. Catalog is fenced below its HTTP
layer: its owner filters go through userscope, it reads no other unit's tables, and its ingest
jobs commit only where they are named.

backend/app/reader/queries.py

@@ -10,9 +10,10 @@ than joining Reader's tables itself.
 from sqlalchemy import ColumnElement, and_, func, or_, select
 from sqlalchemy.orm import InstrumentedAttribute, Session, aliased
 
-from app.feedquery import feed_columns
+from app.collation import collated
+from app.feedquery import feed_columns, serialize
 from app.models import User
-from app.catalog.models import Channel, Video
+from app.catalog.models import Channel, Subscription, Video
 from app.reader.models import ChannelTag, Playlist, PlaylistItem, SavedView, Tag, VideoState
 from app.userscope import owned_by, owned_row
 
@@ -269,3 +270,92 @@ def tags_in_feed_views(db: Session) -> set[int]:
         for (filters,) in db.execute(select(SavedView.filters).where(SavedView.source == "feed"))
         for t in view_tag_ids(filters)
     }
+
+
+def held_by(db: Session, user: User, video_ids: list[str]) -> set[str]:
+    """Which of these videos the user already holds: watched, in progress or hidden, or in one of
+    their playlists (a live search sinks them below the new finds)."""
+    if not video_ids:
+        return set()
+    has_state = (
+        select(VideoState.id)
+        .where(VideoState.video_id == Video.id, owned_by(VideoState.user_id, user))
+        .exists()
+    )
+    in_playlist = (
+        select(PlaylistItem.id)
+        .join(Playlist, Playlist.id == PlaylistItem.playlist_id)
+        .where(PlaylistItem.video_id == Video.id, owned_by(Playlist.user_id, user))
+        .exists()
+    )
+    return set(
+        db.scalars(select(Video.id).where(Video.id.in_(video_ids), or_(has_state, in_playlist)))
+    )
+
+
+def feed_rows_by_ids(db: Session, user: User, video_ids: list[str]) -> list[dict]:
+    """These videos serialized with the feed's projection (their channel, this user's watch state,
+    saved), in the order given."""
+    if not video_ids:
+        return []
+    state = aliased(VideoState)
+    rows = db.execute(
+        select(*feed_columns(user, state))
+        .join(Channel, Channel.id == Video.channel_id)
+        .outerjoin(state, and_(state.video_id == Video.id, owned_by(state.user_id, user)))
+        .where(Video.id.in_(video_ids))
+    ).all()
+    by_id = {r.id: serialize(r) for r in rows}
+    return [by_id[i] for i in video_ids if i in by_id]
+
+
+def playlist_channels(db: Session, user: User, exclude: str | None, lang: str) -> list:
+    """The channels of the videos in the user's playlists that they don't subscribe to (`exclude`:
+    their own channel), each with how many of those videos it has and across how many playlists;
+    most present first, then by title in the reader's alphabet."""
+    subscribed = select(Subscription.channel_id).where(owned_by(Subscription.user_id, user))
+    q = (
+        select(
+            Channel,
+            func.count(func.distinct(PlaylistItem.video_id)),
+            func.count(func.distinct(Playlist.id)),
+        )
+        .select_from(PlaylistItem)
+        .join(Playlist, Playlist.id == PlaylistItem.playlist_id)
+        .join(Video, Video.id == PlaylistItem.video_id)
+        .join(Channel, Channel.id == Video.channel_id)
+        .where(owned_by(Playlist.user_id, user), Channel.id.not_in(subscribed))
+    )
+    if exclude:
+        q = q.where(Channel.id != exclude)
+    return list(
+        db.execute(
+            q.group_by(Channel.id).order_by(
+                func.count(func.distinct(PlaylistItem.video_id)).desc(),
+                collated(Channel.title, lang),
+            )
+        ).all()
+    )
+
+
+def user_tag_links(db: Session, user: User, channel_id: str | None = None) -> dict[str, list[int]]:
+    """The user's own tag links by channel (one channel's, when given)."""
+    stmt = select(ChannelTag.channel_id, ChannelTag.tag_id).where(
+        owned_by(ChannelTag.user_id, user)
+    )
+    if channel_id is not None:
+        stmt = stmt.where(ChannelTag.channel_id == channel_id)
+    out: dict[str, list[int]] = {}
+    for cid, tag_id in db.execute(stmt).tuples():
+        out.setdefault(cid, []).append(tag_id)
+    return out
+
+
+def user_tag_link(db: Session, user: User, channel_id: str, tag_id: int) -> ChannelTag | None:
+    return db.execute(
+        select(ChannelTag).where(
+            ChannelTag.channel_id == channel_id,
+            ChannelTag.tag_id == tag_id,
+            owned_by(ChannelTag.user_id, user),
+        )
+    ).scalar_one_or_none()

backend/app/reader/service.py

@@ -464,3 +464,17 @@ def reorder_items(db: Session, pl: Playlist, order: list[str]) -> int:
     touch(pl)
     db.flush()
     return pos
+
+
+def attach_user_tag(db: Session, user: User, channel_id: str, tag_id: int) -> None:
+    """The user's own label on a channel (idempotent). The caller has checked the tag is theirs."""
+    if queries.user_tag_link(db, user, channel_id, tag_id) is None:
+        db.add(ChannelTag(channel_id=channel_id, tag_id=tag_id, user_id=user.id, source="user"))
+        db.flush()
+
+
+def detach_user_tag(db: Session, user: User, channel_id: str, tag_id: int) -> None:
+    row = queries.user_tag_link(db, user, channel_id, tag_id)
+    if row is not None:
+        db.delete(row)
+        db.flush()