generate_preference_list: Use set instead of recalling object_gen

- removes advanced feature of listing blocked videos ahead
  - was not practical for UI as videos were displayed which "require" others
- using a set to manage still to consider videos makes method way faster

server/entertainment_decider/models.py

@@ -395,7 +395,7 @@ def generate_preference_list(
     score_adapt: float,
     limit: Optional[int] = None,
 ) -> List[MediaElement]:
-    element_list = object_gen()
+    element_list = set(object_gen())
 
     # add tags corresponding to collections
     collections: Iterable[MediaCollection] = MediaCollection.select()
@@ -419,9 +419,8 @@ def generate_preference_list(
         res_ids.append(first_element.id)
         if limit is not None and limit <= len(res_ids):
             break
-        first_element.watched = True  # simulative
+        element_list.remove(first_element)
         base = base.adapt_score(first_element, score_adapt)
-        element_list = object_gen()
     orm.rollback()
     db.execute(f"ALTER TABLE {Tag._table_} AUTO_INCREMENT = 1;")
     return [MediaElement[i] for i in res_ids]