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/components/PlexInfo.tsx

@@ -6,6 +6,7 @@ import {
   Check,
   ExternalLink,
   FolderPlus,
+  ListPlus,
   Play,
   RotateCcw,
   SlidersHorizontal,
@@ -14,6 +15,7 @@ import {
 } from "lucide-react";
 import { api, type PlexFilters, type PlexItemDetail } from "../lib/api";
 import PlexCollectionEditor from "./PlexCollectionEditor";
+import PlexPlaylistAdd from "./PlexPlaylistAdd";
 
 // The rich "media info" surface for a Plex item — poster/art, IMDb score + link, content rating,
 // genres, director, cast (with photos), summary. Reused in TWO places: a full-page view opened from
@@ -95,6 +97,7 @@ export default function PlexInfo({
   };
   const isAdmin = (qc.getQueryData<{ role?: string }>(["me"])?.role) === "admin";
   const [editorOpen, setEditorOpen] = useState(false);
+  const [playlistOpen, setPlaylistOpen] = useState(false);
   const [customizing, setCustomizing] = useState(false);
   const custBtnRef = useRef<HTMLButtonElement>(null);
   const custMenuRef = useRef<HTMLDivElement>(null);
@@ -367,6 +370,15 @@ export default function PlexInfo({
                     {t("plex.info.clearResume")}
                   </button>
                 )}
+                {detail.kind === "movie" && (
+                  <button
+                    onClick={() => setPlaylistOpen(true)}
+                    className="glass-card glass-hover inline-flex items-center gap-1.5 rounded-xl px-3 py-2 text-sm hover:text-accent"
+                  >
+                    <ListPlus className="w-4 h-4" />
+                    {t("plex.playlistAdd.manage")}
+                  </button>
+                )}
                 {isAdmin && detail.kind === "movie" && library && (
                   <button
                     onClick={() => setEditorOpen(true)}
@@ -513,6 +525,9 @@ export default function PlexInfo({
           onChanged={() => onStateChange?.()}
         />
       )}
+      {playlistOpen && (
+        <PlexPlaylistAdd item={{ id: detail.id, title: detail.title }} onClose={() => setPlaylistOpen(false)} />
+      )}
     </div>
   );
 }