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/en/plex.json

@@ -53,6 +53,7 @@
     "errGeneric": "Playback couldn't start. Please try again.",
     "stop": "Stop",
     "help": "Keyboard shortcuts",
+    "infoBtn": "Media info",
     "keys": {
       "playPause": "Play / pause",
       "seek": "Seek 10 seconds",
@@ -60,10 +61,23 @@
       "volume": "Volume up / down",
       "mute": "Mute",
       "fullscreen": "Fullscreen",
+      "info": "Media info",
       "back": "Stop & back to feed",
       "help": "Show this help"
     }
   },
+  "info": {
+    "title": "Media info",
+    "customize": "Customize view",
+    "prefArtBg": "Faint backdrop",
+    "prefCast": "Cast & crew",
+    "openImdb": "Open on IMDb",
+    "director": "Director",
+    "cast": "Cast & crew",
+    "markWatched": "Mark watched",
+    "markUnwatched": "Mark unwatched",
+    "clearResume": "Clear resume position"
+  },
   "playable": {
     "direct": "Plays directly in the browser",
     "remux": "Needs a light remux (no video re-encode)",