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/core/ids.py

@@ -52,6 +52,8 @@ ENTITY_PREFIXES: dict[str, str] = {
     "release": "rel",
     "watch_provider": "watch_provider",
     "user": "user",
+    "rating": "rating",
+    "watchlist_entry": "watchlist",
 }