feat(plex): search cast & crew names + clickable person cards

Backend (migration 0046_plex_people_search): a new people_text column (cast +
director names, de-duped) is folded into the generated search_vector at weight B,
so the top search box finds titles by an actor/director name and ranks them above
summary-only mentions. New /api/plex/people endpoint returns the cast/crew matching
the term (name prefix or word-start) with a film count + a headshot (pulled from one
representative film's live metadata; image bytes disk-cached).

Frontend: PlexBrowse shows matching people as virtual cards above the grid; clicking
one adds them to the actor/director filter (multi-value, from the C1 work) and clears
the search box so you land on exactly that person's films. Answers the 'why does
"drew" match Rambo?' confusion — it was matching the word in the synopsis; now names
are searchable. i18n en/hu/de.

⚠️ Prod needs a Plex re-sync after deploy to populate people_text (search_vector
regenerates automatically once the column has data).

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

@@ -49,6 +49,13 @@
   "markUnwatched": "Mark unwatched",
   "seasons": "{{count}} seasons",
   "playerSoon": "Player coming soon — “{{title}}”",
+  "people": {
+    "match": "People",
+    "actor": "Actor",
+    "director": "Director",
+    "count": "{{count}} titles",
+    "added": "added"
+  },
   "player": {
     "loading": "Loading…",
     "back": "Back",