feat(plex): Plex-web-style 3-level series view (Phase 2 of series view)

Restructure TV browsing into show detail → seasons → season episodes → player, like
the Plex web app.

Backend: /show/{rk} now returns a rich show page — hero meta (rating/content_rating/
genres/studio + live IMDb), live Cast & Crew, Related shows (Plex 'related', mapped to
our mirrored shows so they're openable), and per-season cards with an aggregate
watch-state + on-deck episode, plus show-level resume (on-deck)/first(play-from-start)/
status rollup. New PlexClient.related(). New bulk-state endpoints POST /show/{rk}/state
and /season/{rk}/state mark every episode watched/unwatched for the user and mirror
each change to a linked Plex account in the background (best-effort, checked once).

Frontend: PlexShowView reworked into the show detail page (hero + Resume/Play-from-start/
Mark-show-watched/Add-to-playlist/[admin]Add-to-collection + season card grid + cast +
related strips); new PlexSeasonView season subpage (hero + Resume/Play/Mark-season/
Add-season-to-playlist + landscape episode grid). Both read the one cached ['plex-show']
payload (season page picks its season out of it — instant, no extra fetch). Player queue =
the whole show (from the show page) or the season (from the season page) so prev/next +
auto-advance follow order. New 'season' history subview; Backspace steps back one drill
level (grid←show←season, out of info/playlist) alongside browser/mouse Back. Season-level
'Add to collection' intentionally omitted (Plex collections hold whole shows, not seasons).
i18n plex.series.* (en/hu/de).

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

@@ -70,6 +70,18 @@
   "markWatched": "Als gesehen markieren",
   "markUnwatched": "Als ungesehen markieren",
   "seasons": "{{count}} Staffeln",
+  "series": {
+    "seasons": "Staffeln",
+    "backToShow": "Zurück zur Serie",
+    "episodeCount": "{{count}} Folgen",
+    "playFromStart": "Von Anfang an",
+    "related": "Ähnliche Serien",
+    "markShowWatched": "Serie als gesehen",
+    "markShowUnwatched": "Serie als ungesehen",
+    "markSeasonWatched": "Staffel als gesehen",
+    "markSeasonUnwatched": "Staffel als ungesehen",
+    "addShowCollection": "Zur Sammlung"
+  },
   "playerSoon": "Player kommt bald — „{{title}}“",
   "people": {
     "match": "Personen",