feat(plex): Playlists Phase 1 — per-user ordered watch-lists (Siftlode-native)

Personal ordered lists of Plex items, kept in Siftlode's own DB (works for users
without a Plex account, like watch-state) — the "your own lists" counterpart to
shared collections. Plex-direction sync is a later phase (plex_rating_key
reserved).

Backend: migration 0048 (plex_playlists + plex_playlist_items with position),
PlexPlaylist/PlexPlaylistItem models, and per-user CRUD endpoints under
/api/plex/playlists (list [+?contains for the add dialog], create [seeded],
detail [ordered cards], rename, delete, add/remove item, reorder). _leaf_card
handles the movie/episode mix. Frontend: "Add to playlist" dialog from the movie
info page (all users), a Playlists section in PlexSidebar (list + create),
PlexPlaylistView (reorder up/down, remove, rename, delete, Play all), and
PlexPlayer gained an optional `queue` so play-through follows the list order
(prev/next + auto-advance). i18n en/hu/de. Verified end-to-end on localdev
(backend CRUD + the create→add→view→play-through UI flow).

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

@@ -151,5 +151,27 @@
     "direct": "Plays directly in the browser",
     "remux": "Needs a light remux (no video re-encode)",
     "transcode": "Needs transcoding (heavier)"
+  },
+  "playlistAdd": {
+    "manage": "Add to playlist",
+    "title": "Add to playlist — {{title}}",
+    "newPlaceholder": "New playlist name…",
+    "create": "Create",
+    "none": "No playlists yet. Create one above.",
+    "count": "{{count}} items"
+  },
+  "playlist": {
+    "section": "Playlists",
+    "newPlaceholder": "New playlist…",
+    "create": "Create playlist",
+    "none": "No playlists yet.",
+    "deleteConfirm": "Delete the playlist \"{{title}}\"?",
+    "rename": "Rename",
+    "playAll": "Play all",
+    "delete": "Delete playlist",
+    "empty": "This playlist is empty. Add titles from their info page.",
+    "up": "Move up",
+    "down": "Move down",
+    "remove": "Remove"
   }
 }