feat(plex): unify movies + shows into one cross-library browser

Collapse the separate Movie/Show library sections into ONE unified library with a
shared search + shared filter sidebar and a Movies/Shows/Both scope selector.

Backend: new GET /api/plex/library — a cross-library UNION of movies (plex_items) and
shows (plex_shows) as one mixed, paginated, sorted feed, scoped movie|show|both, with
the shared filters (extracted into _apply_meta_filters, DRY), FTS search, and per-user
watch-state (a show's state = the aggregate of its episodes). On a search that also
matches episodes, matching episodes come back in a separate 'episodes' list (the grouped
'Episodes' section — Proposal 3). /facets is now scope-aware (merged across the scope's
libraries). /item and /show now return their library section key (for the admin
collection editor, since there's no single library prop in the unified view).

Frontend: PlexSidebar's library picker -> a scope selector (Both/Movies/Shows); facets +
browse follow the scope (App's plexLib repurposed to a validated scope, default both).
PlexBrowse uses the unified endpoint, renders a mixed Titles grid + an Episodes section
on search. Poster cards are generalized: a hover Play/Resume overlay on every card, a
clickable title (not just the poster), and a movie/show type tag. The quick watched
toggle is now optimistic so it reliably flips BOTH ways (fixes the movie card that could
mark but not un-mark). Cast/crew members and the show hero's meta (year/rating/genre/
content-rating) are clickable filters; clicking a person widens the scope to Both so the
result is a mixed movie+show feed. i18n plex.filter.scope*/plex.unified.* (en/hu/de).

Still pending from the polish list (next pass): season-card quick toggles (watched +
add-to-playlist), per-episode watched toggle, and the full glassy art-bg + hide-cast
customize menu on the series pages.

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

@@ -13,6 +13,12 @@
   },
   "filter": {
     "library": "Bibliothek",
+    "scope": "Bibliothek",
+    "scopeOpt": {
+      "both": "Alle",
+      "movie": "Filme",
+      "show": "Serien"
+    },
     "show": "Anzeige",
     "showOpt": {
       "all": "Alle",
@@ -70,6 +76,10 @@
   "markWatched": "Als gesehen markieren",
   "markUnwatched": "Als ungesehen markieren",
   "seasons": "{{count}} Staffeln",
+  "unified": {
+    "titles": "Titel",
+    "episodes": "Folgen"
+  },
   "series": {
     "seasons": "Staffeln",
     "backToShow": "Zurück zur Serie",