fix(plex): address S4 review — cap filmography, dedupe toggle, scroll guard, tests

- Efficiency: cap each person section at _PERSON_SECTION_MAX (200) so a prolific person can't
  return an unbounded list the client mounts a card per row for.
- Reuse: extract the optimistic watch-toggle (flip → persist → resync) into a shared
  toggleWatchState helper; the grid and the person page each supply their own cache updater.
- Correctness (scroll): onOpenPerson now stashes infoScrollRef ONLY when leaving an info page
  (not from a show/grid context), and a jumped-to info page resets it — so a stale offset can't
  wrongly restore a fresh info page.
- Test-coverage: add PlexPersonView.dom.test.tsx (export CastStrip + PlexPersonView) covering
  the cast-row rewiring (open-person vs filter fallback) and the person page's sections + filter
  + empty state.

The name-as-identity finding is inherent to Plex cast carrying no per-person id (PlexPerson is
name-keyed too); documented in the endpoint docstring, not structurally fixable in S4.

backend/app/routes/plex/browse.py

@@ -1607,6 +1607,12 @@ def people_cards(
     return {"people": out}
 
 
+# Cap each filmography section — a prolific person shouldn't return an unbounded list the client then
+# mounts a card per row for (the grid paginates at PAGE; this page renders all at once). Ample for a
+# personal library; the most-recent titles win via the year-desc order.
+_PERSON_SECTION_MAX = 200
+
+
 @router.get("/person/{name}")
 def person_detail(
     name: str,
@@ -1615,8 +1621,9 @@ def person_detail(
 ) -> dict:
     """One cast/crew member's filmography: their cached headshot + the movies and shows (in the enabled
     libraries) whose cast OR directors include this name — the inverse of the cast-name → library-filter
-    link. The name string is the identity key (Plex cast carries no stable per-person id). An unknown
-    name simply returns empty sections."""
+    link. The name string is the identity key (Plex cast carries no stable per-person id), so distinct
+    people who share a name share a page. Each section is capped at _PERSON_SECTION_MAX. An unknown name
+    simply returns empty sections."""
     photo = db.scalar(select(PlexPerson.photo_thumb).where(PlexPerson.name == name))
     movie_lib_ids, show_lib_ids = _content_libs(db)
 
@@ -1633,6 +1640,7 @@ def person_detail(
                 ),
             )
             .order_by(PlexItem.year.desc().nullslast(), func.lower(PlexItem.title))
+            .limit(_PERSON_SECTION_MAX)
         ).scalars().all()
         item_ids = [it.id for it in items]
         states = {
@@ -1658,6 +1666,7 @@ def person_detail(
                 ),
             )
             .order_by(PlexShow.year.desc().nullslast(), func.lower(PlexShow.title))
+            .limit(_PERSON_SECTION_MAX)
         ).scalars().all()
         statuses = rollup.state_map(db, user.id, [s.id for s in show_rows])
         shows = [_show_card(sh, statuses.get(sh.id, "new")) for sh in show_rows]