perf(plex): the facets ask the episode rollup once per call, not once per show group

A watch-state scope (watched / in progress / unwatched) joined the per-user episode rollup into
each show facet query: five passes over the user's episodes per call, half of a filtered facets
request. The shows in scope are now resolved once and every group filters on their ids; the
grid-parity test holds the counts. Localdev: 99 -> 61 ms.

backend/tests/test_plex_facets_grid_parity.py

@@ -214,3 +214,28 @@ def test_hidden_movie_absent_from_grid_and_facets(db, as_user):
     facets = client.get("/api/plex/facets?scope=movie&show=all")
     crs = {c["value"]: c["count"] for c in facets.json()["content_ratings"]}
     assert crs.get("R") == 1, crs
+
+
+def test_a_watch_state_scope_rolls_the_episodes_up_once(db, as_user):
+    """The per-user episode rollup behind a show's watch state is a pass over every episode; each show
+    facet group asked it again (E31 S4e: half of a filtered facets call). It is asked once now."""
+    from sqlalchemy import event
+
+    from app.db import engine
+
+    seed = _seed(db)
+    client = as_user(seed["user"])
+    rollups: list[str] = []
+
+    def count(conn, cursor, statement, *_):
+        if "GROUP BY plex_items.show_id" in statement:
+            rollups.append(statement)
+
+    event.listen(engine, "before_cursor_execute", count)
+    try:
+        for show in ("watched", "in_progress", "unwatched"):
+            rollups.clear()
+            assert client.get(f"/api/plex/facets?scope=both&show={show}").status_code == 200
+            assert len(rollups) == 1, show
+    finally:
+        event.remove(engine, "before_cursor_execute", count)