feat(playlists): backend foundation — model, migration, CRUD API

Add per-user local playlists: Playlist + PlaylistItem models (with forward-looking
kind/source/yt_playlist_id/dirty columns for the later YouTube-sync phases) and
migration 0011. New /api/playlists routes: list (with optional contains=<video_id>
membership flags for the add-to-playlist popover), create, get detail (items
serialized via the feed serializer, with per-user watch state), rename, delete,
add/remove item, and reorder. Watch later (built-in) is protected from deletion.

backend/alembic/versions/0011_playlists.py

@@ -0,0 +1,88 @@
+"""local playlists: playlists + playlist_items
+
+Revision ID: 0011_playlists
+Revises: 0010_watch_progress
+Create Date: 2026-06-15
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0011_playlists"
+down_revision: Union[str, None] = "0010_watch_progress"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "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("name", sa.String(length=255), nullable=False),
+        sa.Column(
+            "kind", sa.String(length=16), nullable=False, server_default="user"
+        ),
+        sa.Column(
+            "source", sa.String(length=16), nullable=False, server_default="local"
+        ),
+        sa.Column("yt_playlist_id", sa.String(length=64), nullable=True),
+        sa.Column(
+            "dirty", sa.Boolean(), nullable=False, server_default="false"
+        ),
+        sa.Column("position", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.func.now(),
+            nullable=False,
+        ),
+        sa.Column(
+            "updated_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.func.now(),
+            nullable=False,
+        ),
+    )
+    op.create_index("ix_playlists_user_id", "playlists", ["user_id"])
+
+    op.create_table(
+        "playlist_items",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column(
+            "playlist_id",
+            sa.Integer(),
+            sa.ForeignKey("playlists.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column(
+            "video_id",
+            sa.String(length=16),
+            sa.ForeignKey("videos.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column("position", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column(
+            "added_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.func.now(),
+            nullable=False,
+        ),
+        sa.UniqueConstraint("playlist_id", "video_id", name="uq_playlist_video"),
+    )
+    op.create_index("ix_playlist_items_playlist_id", "playlist_items", ["playlist_id"])
+    op.create_index("ix_playlist_items_video_id", "playlist_items", ["video_id"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_playlist_items_video_id", table_name="playlist_items")
+    op.drop_index("ix_playlist_items_playlist_id", table_name="playlist_items")
+    op.drop_table("playlist_items")
+    op.drop_index("ix_playlists_user_id", table_name="playlists")
+    op.drop_table("playlists")