feat(player): the YouTube player and the file viewer are named in the address

The player names what it plays in its own history entry (watch, wrel, wtab): a reload or a
bookmark reopens it on the page that hosts it, with that page's list as the queue, or alone
from the catalog (GET /api/videos/{id}/row); a page that hosts none gets the shell's. Back
lands on the page's address without it (useBackToClose's url; ui/Modal passes it on). The
Files viewer and player do the same (fview, fplay). The Plex and channel-manager views are
always written in full, as their fallback is remembered state.

backend/app/routes/feed.py

@@ -249,6 +249,21 @@ def set_video_progress(
     return {"ok": True, "video_id": video_id, "position_seconds": position}
 
 
+@router.get("/videos/{video_id}/row")
+def get_video_row(
+    video_id: str = Path(max_length=32),
+    user: User = Depends(current_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    """One catalog video as the feed lists it (its channel, this user's watch state and resume point,
+    saved): what a player opened from an address starts from. The catalog only, so it costs nothing;
+    every video a player can open (a list's row, a related one) is in it."""
+    rows = queries.feed_rows_by_ids(db, user, [video_id])
+    if not rows:
+        raise not_found("video")
+    return rows[0]
+
+
 @router.get("/videos/{video_id}")
 def get_video_detail(
     video_id: str,

backend/tests/test_video_row.py

@@ -0,0 +1,47 @@
+"""A catalog video as the feed lists it, the row a player opened from an address starts from. DB lane."""
+
+from datetime import datetime, timezone
+
+from app.catalog.models import Channel, Video
+from app.reader.models import VideoState
+
+
+def _video(db, vid="v1"):
+    db.add(Channel(id="UCo", title="Owner", handle="@owner"))
+    db.flush()
+    db.add(
+        Video(
+            id=vid,
+            title="A talk",
+            channel_id="UCo",
+            published_at=datetime.now(timezone.utc),
+            duration_seconds=3600,
+        )
+    )
+    db.commit()
+
+
+def test_a_video_comes_as_the_feed_row_with_my_own_state(db, make_user, as_user):
+    me, other = make_user(), make_user()
+    _video(db)
+    db.add(VideoState(user_id=me.id, video_id="v1", status="new", position_seconds=600))
+    db.add(VideoState(user_id=other.id, video_id="v1", status="watched", position_seconds=90))
+    db.commit()
+    r = as_user(me).get("/api/videos/v1/row")
+    assert r.status_code == 200, r.text
+    row = r.json()
+    assert (row["id"], row["title"], row["channel_title"]) == ("v1", "A talk", "Owner")
+    assert (row["status"], row["position_seconds"]) == ("new", 600)
+    assert row["watch_url"] == "https://www.youtube.com/watch?v=v1"
+
+
+def test_a_video_never_opened_has_no_state(db, make_user, as_user):
+    _video(db)
+    row = as_user(make_user()).get("/api/videos/v1/row").json()
+    assert (row["status"], row["position_seconds"], row["saved"]) == ("new", 0, False)
+
+
+def test_a_video_outside_the_catalog_is_not_found(db, make_user, as_user):
+    r = as_user(make_user()).get("/api/videos/nope/row")
+    assert r.status_code == 404
+    assert r.json()["detail"]["code"] == "not_found"