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/app/models.py

@@ -301,3 +301,64 @@ class AppState(Base):
     sync_paused: Mapped[bool] = mapped_column(
         Boolean, default=False, server_default="false"
     )
+
+
+class Playlist(Base):
+    """A per-user named, ordered collection of videos from the shared catalog.
+
+    `kind`: 'user' (normal) or 'watch_later' (built-in, undeletable, one per user).
+    `source`: 'local' (created here) or 'youtube' (mirrored from the user's YouTube
+    playlists). `yt_playlist_id` links a mirrored/exported playlist to its YouTube id.
+    `dirty` marks a YouTube-linked playlist with local edits not yet pushed, so the read
+    sync won't clobber them. (source/yt_playlist_id/dirty are forward-looking for the YT
+    sync phases; the foundation phase only uses local playlists.)"""
+
+    __tablename__ = "playlists"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"), index=True
+    )
+    name: Mapped[str] = mapped_column(String(255))
+    kind: Mapped[str] = mapped_column(String(16), default="user", server_default="user")
+    source: Mapped[str] = mapped_column(
+        String(16), default="local", server_default="local"
+    )
+    yt_playlist_id: Mapped[str | None] = mapped_column(String(64))
+    dirty: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
+    position: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
+    )
+
+    items: Mapped[list["PlaylistItem"]] = relationship(
+        back_populates="playlist",
+        cascade="all, delete-orphan",
+        order_by="PlaylistItem.position",
+    )
+
+
+class PlaylistItem(Base):
+    """A video's membership in a playlist, with its order within that playlist."""
+
+    __tablename__ = "playlist_items"
+    __table_args__ = (
+        UniqueConstraint("playlist_id", "video_id", name="uq_playlist_video"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    playlist_id: Mapped[int] = mapped_column(
+        ForeignKey("playlists.id", ondelete="CASCADE"), index=True
+    )
+    video_id: Mapped[str] = mapped_column(
+        ForeignKey("videos.id", ondelete="CASCADE"), index=True
+    )
+    position: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
+    added_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+
+    playlist: Mapped["Playlist"] = relationship(back_populates="items")