Add movie/series ratings and watchlist

Builds on the accounts foundation: PUT/DELETE a rating (0.5-10.0) or a
watchlist entry on any movie or series, all scoped to the bearer token's
own user, plus GET /account/ratings|watchlist/{movies,series} to list
them back. Both are polymorphic entity_type/entity_id tables, same shape
as watch_provider_offerings and trending_entries, so movie and series
share one repository/router each rather than duplicating per type.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

app/models/watchlist.py

@@ -0,0 +1,30 @@
+"""User watchlists -- section 17, additive (accounts/ratings/watchlist).
+
+Polymorphic across movie/series, same shape as Rating -- no value beyond
+membership, so no extra columns beyond the timestamps."""
+
+from __future__ import annotations
+
+import uuid
+
+from sqlalchemy import ForeignKey, Index, String, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+
+
+class WatchlistEntry(Base, UUIDPrimaryKeyMixin, TimestampMixin):
+    __tablename__ = "watchlist_entries"
+    __table_args__ = (
+        UniqueConstraint("user_id", "entity_type", "entity_id"),
+        Index("ix_watchlist_entries_user", "user_id", "entity_type"),
+    )
+
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"), nullable=False
+    )
+    entity_type: Mapped[str] = mapped_column(String(30), nullable=False)
+    entity_id: Mapped[uuid.UUID] = mapped_column(nullable=False)
+
+
+__all__ = ["WatchlistEntry"]