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).

backend/app/models.py

@@ -1109,3 +1109,33 @@ class PlexState(Base, UpdatedAtMixin):
     synced_to_plex: Mapped[bool] = mapped_column(
         Boolean, default=False, server_default="false"
     )
+
+
+class PlexPlaylist(Base, TimestampMixin, UpdatedAtMixin):
+    """A per-user ORDERED watch-list of Plex items — Siftlode-native (lives in our own DB, works for
+    users WITHOUT a Plex account, like plex_states). Unlike collections (shared, library-level), a
+    playlist is owned by one user and its items are ordered. Optional Plex-direction sync is a later
+    phase (`plex_rating_key` set once pushed back to the owner's Plex account)."""
+
+    __tablename__ = "plex_playlists"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), index=True)
+    title: Mapped[str] = mapped_column(String(512))
+    plex_rating_key: Mapped[str | None] = mapped_column(String(32))
+
+
+class PlexPlaylistItem(Base):
+    """One ordered entry in a playlist. `position` (0-based) orders playback."""
+
+    __tablename__ = "plex_playlist_items"
+    __table_args__ = (UniqueConstraint("playlist_id", "item_id", name="uq_plex_playlist_item"),)
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    playlist_id: Mapped[int] = mapped_column(
+        ForeignKey("plex_playlists.id", ondelete="CASCADE"), index=True
+    )
+    item_id: Mapped[int] = mapped_column(
+        ForeignKey("plex_items.id", ondelete="CASCADE"), index=True
+    )
+    position: Mapped[int] = mapped_column(Integer, default=0, server_default="0", nullable=False)