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/i18n/locales/en/plex.json

@@ -203,6 +203,7 @@
     "filterYear": "Show titles from {{year}}",
     "filterRating": "Show titles rated {{n}}+",
     "filterActor": "Show titles with {{name}}",
+    "viewPerson": "View {{name}}'s filmography",
     "browseCollection": "Browse collection →"
   },
   "playable": {
@@ -245,5 +246,12 @@
     "count_one": "{{count}} title",
     "count_other": "{{count}} titles",
     "remove": "Remove from filter"
+  },
+  "person": {
+    "credits": "{{movies}} movies · {{shows}} shows",
+    "filterBy": "Filter library by {{name}}",
+    "movies": "Movies ({{count}})",
+    "shows": "Shows ({{count}})",
+    "empty": "No titles found for this person."
   }
 }

frontend/src/i18n/locales/hu/plex.json

@@ -203,6 +203,7 @@
     "filterYear": "{{year}} évi címek",
     "filterRating": "{{n}}+ pontos címek",
     "filterActor": "Címek {{name}} szereplésével",
+    "viewPerson": "{{name}} filmográfiája",
     "browseCollection": "Kollekció böngészése →"
   },
   "playable": {
@@ -245,5 +246,12 @@
     "count_one": "{{count}} cím",
     "count_other": "{{count}} cím",
     "remove": "Eltávolítás a szűrőből"
+  },
+  "person": {
+    "credits": "{{movies}} film · {{shows}} sorozat",
+    "filterBy": "Szűrés erre: {{name}}",
+    "movies": "Filmek ({{count}})",
+    "shows": "Sorozatok ({{count}})",
+    "empty": "Nincs találat ehhez a személyhez."
   }
 }