feat(plex): S4 actor detail page — filmography endpoint + person subview

Backend: new GET /api/plex/person/{name} returns a cast/crew member's cached headshot plus
the movies (PlexItem, kind=movie) and shows (PlexShow) in the enabled libraries whose
cast_names OR directors include the name — the inverse of the cast → filter link. Introduce a
shared _content_libs(db) helper (enabled-library partition) and reuse it in people_cards;
S5 will extend it with the HU-overlay exclusion. Watch-state comes from PlexState / rollup
(reusing _movie_card / _show_card / state_map). DB-lane tests cover cast+director credits,
library scoping, the kind filter, unknown-name empties, and the partition helper.

Frontend: a new `person` history-subview. A cast-row click (PlexInfo cast row + the show
page's CastStrip) now opens the person's filmography page instead of filtering; the filter
gesture moves to a "Filter library by {name}" button on the page itself. PlexPersonView
mirrors PlexInfoView's fetch-wrapper + glass hero (headshot + name + credit counts) and
renders Movies / Shows as PlexPosterCard grids, with an optimistic per-card watch toggle on
the person cache. New plexPerson query key + api client + PlexPersonDetail type; HU/EN copy.

frontend/src/lib/api/plex.ts

@@ -181,6 +181,12 @@ export interface PlexPersonCard {
   photo?: string | null; // proxied person-image url, or null (placeholder)
   count: number; // in-scope movies/shows featuring them
 }
+export interface PlexPersonDetail {
+  name: string;
+  photo?: string | null; // cached headshot, or null (placeholder)
+  movies: PlexCard[]; // enabled-library movies whose cast OR directors include this person
+  shows: PlexCard[]; // enabled-library shows, same
+}
 // The expanded Plex browse filters (movie libraries). Persisted per-account as JSON.
 export interface PlexFilters {
   genres: string[];
@@ -414,6 +420,9 @@ export const plexApi = {
     req(
       `/api/plex/people/cards?scope=${encodeURIComponent(scope)}&names=${encodeURIComponent(names.join(","))}`,
     ),
+  // One cast/crew member's filmography: cached headshot + movies and shows (cast OR directors).
+  plexPerson: (name: string): Promise<PlexPersonDetail> =>
+    req(`/api/plex/person/${encodeURIComponent(name)}`),
   plexSession: (
     id: string,
     start = 0,