feat(plex): rich media info — info page, in-player info overlay, IMDb + cast photos

Backend (no migration): item_detail now also returns IMDb score + id/url (from the
Plex Rating/Guid arrays), content rating, genres, director(s), studio, tagline, and
cast as {name, role, photo}. New host-whitelisted /person-image proxy serves cast
photos from Plex's public metadata CDN (keeps third-party requests off the browser).

Frontend: reusable PlexInfo component in two forms — a full info page opened from a
card's 'i' button (history subview, art backdrop, Play/Resume + watch controls) and a
lean overlay in the player toggled with 'I' (video keeps playing). Two per-user view
prefs (faint backdrop, cast row) persisted in preferences. Mark-unwatched / clear-resume
cover the watched-reset gap. i18n en/hu/de.

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

@@ -53,6 +53,7 @@
     "errGeneric": "Die Wiedergabe konnte nicht gestartet werden. Bitte erneut versuchen.",
     "stop": "Stopp",
     "help": "Tastenkürzel",
+    "infoBtn": "Medieninfo",
     "keys": {
       "playPause": "Wiedergabe / Pause",
       "seek": "10 Sekunden spulen",
@@ -60,10 +61,23 @@
       "volume": "Lautstärke lauter / leiser",
       "mute": "Stumm",
       "fullscreen": "Vollbild",
+      "info": "Medieninfo",
       "back": "Stopp & zurück zum Feed",
       "help": "Diese Hilfe anzeigen"
     }
   },
+  "info": {
+    "title": "Medieninfo",
+    "customize": "Ansicht anpassen",
+    "prefArtBg": "Dezenter Hintergrund",
+    "prefCast": "Besetzung",
+    "openImdb": "Bei IMDb öffnen",
+    "director": "Regie",
+    "cast": "Besetzung & Crew",
+    "markWatched": "Als gesehen markieren",
+    "markUnwatched": "Gesehen zurücknehmen",
+    "clearResume": "Fortsetzungsposition löschen"
+  },
   "playable": {
     "direct": "Spielt direkt im Browser",
     "remux": "Braucht ein leichtes Remux (keine Video-Neucodierung)",