fix discarding ratings

We want to limit the number of movies, not ratings.  By limiting the
number of movie IDs before loading all ratings with those movie IDs
we can achieve this.

unwind/db.py

@@ -217,6 +217,7 @@ async def find_ratings(
                     SELECT * FROM newest_movies
                     UNION ALL  -- using ALL here avoids the reordering of IDs
                     SELECT * FROM unrated_movies
+                    LIMIT :limit_rows
                 )""",
             ]
         )
@@ -238,7 +239,6 @@ async def find_ratings(
         LEFT JOIN {Rating._table} ON {Rating._table}.movie_id={source_table}.movie_id
         -- LEFT JOIN {User._table} ON {User._table}.id={Rating._table}.user_id
         LEFT JOIN {Movie._table} ON {Movie._table}.id={source_table}.movie_id
-        LIMIT :limit_rows
     """
 
     rows = await shared_connection().fetch_all(bindparams(query, values))