feat(plex): expanded metadata filters, clickable info page, image disk cache

Backend (migration 0045_plex_filter_meta): plex_items gains rating (audienceRating
~IMDb), content_rating, studio, originally_available_at, and GIN-indexed genres /
directors / cast_names — all mirrored from the cheap section listing (no per-item API
calls; they also seed a future watch-habit recommender). /browse gains genre / content-
rating / year / rating / duration / added-within / director / actor / studio filters
(@> containment, GIN) + sort by year|rating|duration|release; new /facets endpoint
returns available genres+ratings (with counts) and the year/rating/duration bounds. A
thin on-disk image cache (.plex-img-cache) serves posters/art/cast photos from local
disk after first fetch (~7-14x faster repeat loads).

Frontend: PlexSidebar grows the full filter set (facet-driven genre/age chips, rating
steps, year range inputs, duration buckets, added-within, active people/studio chips,
clear-all); filters persist per-account as one JSON blob. PlexInfo metadata (year,
genre, director, cast, studio, IMDb score) is clickable → sets the matching filter and
returns to the filtered grid (page variant only; the in-player overlay stays read-only
so a stray click can't stop playback). i18n en/hu/de.

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

@@ -5,7 +5,11 @@
   "searchPlaceholder": "Search Plex…",
   "sort": {
     "added": "Recently added",
-    "title": "Title"
+    "title": "Title",
+    "year": "Year",
+    "rating": "IMDb rating",
+    "duration": "Length",
+    "release": "Release date"
   },
   "filter": {
     "library": "Library",
@@ -16,7 +20,18 @@
       "in_progress": "In progress",
       "watched": "Watched"
     },
-    "sort": "Sort"
+    "sort": "Sort",
+    "clearAll": "Clear filters",
+    "active": "Active",
+    "any": "Any",
+    "rating": "IMDb rating",
+    "genre": "Genre",
+    "year": "Year",
+    "duration": "Length",
+    "durationOpt": { "short": "Under 90m", "mid": "90–120m", "long": "Over 2h" },
+    "added": "Added to Plex",
+    "addedOpt": { "24h": "24h", "1w": "1 week", "1m": "1 month", "6m": "6 months", "1y": "1 year" },
+    "contentRating": "Age rating"
   },
   "count": "{{count}} titles",
   "searchCount": "{{count}} matches",
@@ -76,7 +91,10 @@
     "cast": "Cast & crew",
     "markWatched": "Mark watched",
     "markUnwatched": "Mark unwatched",
-    "clearResume": "Clear resume position"
+    "clearResume": "Clear resume position",
+    "filterYear": "Show titles from {{year}}",
+    "filterRating": "Show titles rated {{n}}+",
+    "filterActor": "Show titles with {{name}}"
   },
   "playable": {
     "direct": "Plays directly in the browser",