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/alembic/versions/0048_plex_playlists.py

@@ -0,0 +1,58 @@
+"""Plex playlists (Siftlode-native, per-user, ordered)
+
+Revision ID: 0048_plex_playlists
+Revises: 0047_plex_collections
+Create Date: 2026-07-06
+
+Per-user ORDERED watch-lists of Plex items, kept in Siftlode's own DB so they work for users without a
+Plex account (like plex_states) — the "your own lists" counterpart to the shared, library-level
+collections. `plex_playlists` is owned by a user; `plex_playlist_items` holds the ordered entries
+(`position`). `plex_rating_key` is reserved for an optional later Plex-direction sync.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0048_plex_playlists"
+down_revision: Union[str, None] = "0047_plex_collections"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "plex_playlists",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column("user_id", sa.Integer(), sa.ForeignKey("users.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("title", sa.String(length=512), nullable=False),
+        sa.Column("plex_rating_key", sa.String(length=32), nullable=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.text("now()"), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), server_default=sa.text("now()"), nullable=False),
+    )
+    op.create_index("ix_plex_playlists_user_id", "plex_playlists", ["user_id"])
+
+    op.create_table(
+        "plex_playlist_items",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column(
+            "playlist_id", sa.Integer(),
+            sa.ForeignKey("plex_playlists.id", ondelete="CASCADE"), nullable=False,
+        ),
+        sa.Column(
+            "item_id", sa.Integer(),
+            sa.ForeignKey("plex_items.id", ondelete="CASCADE"), nullable=False,
+        ),
+        sa.Column("position", sa.Integer(), nullable=False, server_default="0"),
+        sa.UniqueConstraint("playlist_id", "item_id", name="uq_plex_playlist_item"),
+    )
+    op.create_index("ix_plex_playlist_items_playlist_id", "plex_playlist_items", ["playlist_id"])
+    op.create_index("ix_plex_playlist_items_item_id", "plex_playlist_items", ["item_id"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_plex_playlist_items_item_id", table_name="plex_playlist_items")
+    op.drop_index("ix_plex_playlist_items_playlist_id", table_name="plex_playlist_items")
+    op.drop_table("plex_playlist_items")
+    op.drop_index("ix_plex_playlists_user_id", table_name="plex_playlists")
+    op.drop_table("plex_playlists")