feat(player): a video's related videos, on demand beside it

The player shows what YouTube lists beside the video, without what the
user has watched or hidden. Wide, a Related tab beside the description;
stacked, a button beside the info opens them in a drawer. A row plays in
the same player (Back returns) and the list stays the opened video's.
A switch picks on-topic or Hungarian-trending lists, kept on the account.

Each related row says whether the user follows its channel; the row is
the video card's narrow form with its whole menu. VideoMenu moves to
features/video, where the player can reach it.

backend/app/catalog/queries.py

@@ -158,6 +158,19 @@ def blocked_ids(db: Session, user: User) -> set[str]:
     )
 
 
+def followed_among(db: Session, user: User, channel_ids: set[str]) -> set[str]:
+    """Which of these channels the user subscribes to."""
+    if not channel_ids:
+        return set()
+    return set(
+        db.scalars(
+            select(Subscription.channel_id).where(
+                owned_by(Subscription.user_id, user), Subscription.channel_id.in_(channel_ids)
+            )
+        )
+    )
+
+
 def blocked_channels(db: Session, user: User, lang: str) -> list[Channel]:
     return list(
         db.execute(

backend/app/routes/search.py

@@ -284,8 +284,11 @@ def related_videos(
     # (a cursor) is the same list.
     if cursor is None:
         quota.log_action(user.id, quota.QuotaAction.VIDEOS_RELATED)
+    rows = reader_queries.feed_rows_by_ids(db, user, collected)
+    # A related list mixes channels the user follows with ones they never met: each row says which.
+    followed = queries.followed_among(db, user, {r["channel_id"] for r in rows})
     return {
-        "items": reader_queries.feed_rows_by_ids(db, user, collected),
+        "items": [{**r, "followed": r["channel_id"] in followed} for r in rows],
         "next_cursor": next_cursor,
         "has_more": bool(next_cursor),
     }

backend/tests/test_related_route.py

@@ -12,7 +12,7 @@ import pytest
 from sqlalchemy import select
 
 from app import quota, sysconfig
-from app.catalog.models import BlockedChannel, Channel, Video
+from app.catalog.models import BlockedChannel, Channel, Subscription, Video
 from app.models import QuotaEvent
 from app.reader.models import TakeoutItem, TakeoutKind, VideoState, VideoUnmark
 from app.routes import search as search_routes
@@ -296,3 +296,22 @@ def test_reading_further_is_the_same_list_not_another_one(db, make_user, as_user
     assert client.get(f"/api/search/related/{SOURCE}").status_code == 200
     assert client.get(f"/api/search/related/{SOURCE}?cursor=more").status_code == 200
     assert quota.actions_today(db, user.id, quota.QuotaAction.VIDEOS_RELATED) == 1
+
+
+def test_each_row_says_whether_the_user_follows_its_channel(db, make_user, as_user, pages):
+    user = make_user()
+    stranger = make_user()
+    _known(db, 1, channel="UCfollowed1")
+    _known(db, 2, channel="UCstranger1")
+    db.add(Subscription(user_id=user.id, channel_id="UCfollowed1"))
+    db.add(Subscription(user_id=stranger.id, channel_id="UCstranger1"))
+    db.commit()
+    pages["pages"] = [
+        _page([_item(1, "UCfollowed1"), _item(2, "UCstranger1"), _item(3, "UCnobody01")], None)
+    ]
+    rows = as_user(user).get(f"/api/search/related/{SOURCE}").json()["items"]
+    assert {r["id"]: r["followed"] for r in rows} == {
+        _vid(1): True,
+        _vid(2): False,  # another user's follow is not this one's
+        _vid(3): False,
+    }