fix(plex): a collection kept after a failed refresh holds its seed title

When Plex made the collection and only the read after it failed, the kept row had no members: the
title showed unticked in the collection it had just been added to, the collection empty, and the
name stayed in the field for a retry that could only be refused as taken. The seed's row now names
the collection (child_count 1), and the panel clears the field on that refusal as on success. Also
held: the panel's lines sit beside its list, and a keyboard Retry lands on the first row.

backend/app/routes/plex/collections.py

@@ -470,8 +470,17 @@ def create_collection(
             # It is on Plex now, and only the follow-up read failed. Its row is kept, editable: the
             # full sync preserves `editable` but creates a missing row read-only, so the collection
             # would drop out of every "add to" list; and the duplicate-name check reads these rows,
-            # so a retry of the same name would make a second one on Plex.
-            db.add(PlexCollection(rating_key=new_rk, library_id=lib.id, title=title, editable=True))
+            # so a retry of the same name would make a second one on Plex. Plex made it with exactly
+            # the seed in it, so the seed's row says so too — otherwise the title showed unticked in
+            # the collection it was just added to, and the collection empty, until that sync.
+            db.add(
+                PlexCollection(
+                    rating_key=new_rk, library_id=lib.id, title=title, editable=True, child_count=1
+                )
+            )
+            Model = PlexItem if lib.kind == "movie" else PlexShow
+            if row := by_rating_key(db, Model, seed):
+                row.collection_keys = list(dict.fromkeys([*(row.collection_keys or []), new_rk]))
             db.commit()
         raise _write_or_pending_error(wrote, e) from e
     return _collection_card(col)

backend/tests/test_plex_collections_filter.py

@@ -360,6 +360,9 @@ def test_create_resync_failure_is_422(db, monkeypatch, override_dep):
     # where collections are added to, and the same name is not made twice.
     row = by_rating_key(db, PlexCollection, "NEW")
     assert row is not None and row.editable and row.title == "Fresh Coll"
+    # Plex made it with the seed in it: the seed's row says so, so the title shows ticked in it.
+    assert row.child_count == 1
+    assert "NEW" in (by_rating_key(db, PlexItem, "m1").collection_keys or [])
     again = client.post(
         "/api/plex/collections",
         json={"library": "1", "title": "Fresh Coll", "item_rating_key": "m1"},